diff --git a/lint/lint.go b/lint/lint.go
index eccc867..aa5c9eb 100644
--- a/lint/lint.go
+++ b/lint/lint.go
@@ -63,19 +63,20 @@ type Config struct {
 
 // Rule codes.
 const (
-	CodeUnknownInstr    = "unknown-instruction"
-	CodeOperandCount    = "operand-count"
-	CodeUndefinedLabel  = "undefined-label"
-	CodeDuplicateLabel  = "duplicate-label"
-	CodeMissingRet      = "missing-ret"
-	CodeMissingTextflag = "missing-textflag-include"
-	CodeUnreachable     = "unreachable-code"
-	CodeABIArgSize      = "abi-argsize"
-	CodeRegisterClobber = "register-clobber"
-	CodeFuncdata        = "funcdata-pcdata"
-	CodeUnusedLabel     = "unused-label"
-	CodeInvalidFlag     = "invalid-textflag"
-	CodeStackImbalance  = "stack-imbalance"
+	CodeUnknownInstr          = "unknown-instruction"
+	CodeOperandCount          = "operand-count"
+	CodeUndefinedLabel        = "undefined-label"
+	CodeDuplicateLabel        = "duplicate-label"
+	CodeMissingRet            = "missing-ret"
+	CodeMissingTextflag       = "missing-textflag-include"
+	CodeUnreachable           = "unreachable-code"
+	CodeABIArgSize            = "abi-argsize"
+	CodeRegisterClobber       = "register-clobber"
+	CodeFuncdata              = "funcdata-pcdata"
+	CodeUnusedLabel           = "unused-label"
+	CodeInvalidFlag           = "invalid-textflag"
+	CodeStackImbalance        = "stack-imbalance"
+	CodeRegisterWidthMismatch = "register-width-mismatch"
 )
 
 // knownTextFlags are the flags recognised by the Go assembler's textflag.h.
@@ -299,6 +300,21 @@ func lintText(t *ast.Text, tab *arch.Table, archKnown bool, cfg Config, macros m
 				}
 			}
 
+			// Register-width mismatch: amd64 instructions with Q suffix
+			// should use 64-bit registers, L/W/B suffix should use
+			// 32/16/8-bit registers.
+			if cfg.Arch == arch.AMD64 && !cfg.Disable[CodeRegisterWidthMismatch] && !isMacroInvocation(mnem, macros) {
+				if msg := checkRegisterWidth(upper, st.Operands); msg != "" {
+					out = append(out, Diagnostic{
+						Pos:      st.Mnemonic.Pos,
+						End:      st.Mnemonic.End,
+						Severity: Warning,
+						Code:     CodeRegisterWidthMismatch,
+						Message:  msg,
+					})
+				}
+			}
+
 			if isJump(cfg.Arch, upper) {
 				for _, op := range st.Operands {
 					if name, pos, ok := localLabelRef(op); ok && !tab.IsRegister(name) && !arch.IsPseudoReg(name) {
@@ -714,6 +730,59 @@ func isSPReg(op *ast.Operand, a arch.Arch) bool {
 	return false
 }
 
+// checkRegisterWidth detects amd64 register-width mismatches: a Q-suffix
+// instruction (64-bit) using a 32-bit register, or an L/W/B-suffix
+// instruction using a 64-bit register.
+func checkRegisterWidth(mnem string, ops []*ast.Operand) string {
+	// Determine expected width from mnemonic suffix.
+	var expected int // 0=unknown, 8/4/2/1=bytes
+	switch {
+	case strings.HasSuffix(mnem, "Q"):
+		expected = 8
+	case strings.HasSuffix(mnem, "L"):
+		expected = 4
+	case strings.HasSuffix(mnem, "W"):
+		expected = 2
+	case strings.HasSuffix(mnem, "B"):
+		expected = 1
+	default:
+		return "" // no suffix, can't determine width
+	}
+	for _, op := range ops {
+		if op.Kind != ast.OpAddr || op.Addr.Sym == nil {
+			continue
+		}
+		name := strings.ToLower(op.Addr.Sym.Name)
+		regWidth := amd64RegWidth(name)
+		if regWidth == 0 {
+			continue // not a register or unknown
+		}
+		if expected == 8 && regWidth == 4 {
+			return fmt.Sprintf("%s uses 32-bit register %s (expected 64-bit)", mnem, op.Addr.Sym.Name)
+		}
+		if expected == 4 && regWidth == 8 {
+			return fmt.Sprintf("%s uses 64-bit register %s (expected 32-bit)", mnem, op.Addr.Sym.Name)
+		}
+	}
+	return ""
+}
+
+// amd64RegWidth returns the width in bytes of an amd64 register name.
+func amd64RegWidth(name string) int {
+	switch name {
+	case "rax", "rbx", "rcx", "rdx", "rsi", "rdi", "rbp", "rsp",
+		"r8", "r9", "r10", "r11", "r12", "r13", "r14", "r15":
+		return 8
+	case "eax", "ebx", "ecx", "edx", "esi", "edi", "ebp", "esp":
+		return 4
+	case "ax", "bx", "cx", "dx", "si", "di", "bp", "sp":
+		return 2
+	case "al", "bl", "cl", "dl", "ah", "bh", "ch", "dh":
+		return 1
+	}
+	return 0
+}
+
 func countRange(min, max int) string {
 	if min == max {
 		return fmt.Sprintf("%d operand(s)", min)
diff --git a/lint/lint_test.go b/lint/lint_test.go
index 917fa14..8eb7c50 100644
--- a/lint/lint_test.go
+++ b/lint/lint_test.go
@@ -309,3 +309,29 @@ TEXT ·f(SB), NOSPLIT, $16-0
 		t.Fatalf("balanced stack must not be flagged: %+v", diags)
 	}
 }
+
+func TestRegisterWidthMismatch(t *testing.T) {
+	// MOVQ with 32-bit register — mismatch.
+	diags := lintSrc(t, `
+#include "textflag.h"
+TEXT ·f(SB), NOSPLIT, $0
+	MOVQ EAX, BX
+	RET
+`)
+	if codes(diags)[CodeRegisterWidthMismatch] != 1 {
+		t.Fatalf("want one register-width-mismatch, got %+v", diags)
+	}
+}
+
+func TestRegisterWidthCorrect(t *testing.T) {
+	// MOVQ with 64-bit registers — correct.
+	diags := lintSrc(t, `
+#include "textflag.h"
+TEXT ·f(SB), NOSPLIT, $0
+	MOVQ RAX, RBX
+	RET
+`)
+	if codes(diags)[CodeRegisterWidthMismatch] != 0 {
+		t.Fatalf("correct width must not be flagged: %+v", diags)
+	}
+}