To assess the clinical significance of "giant" negative T waves in patients with hypertrophic cardiomyopathy from Western nations, clinical, echocardiographic, radionuclide and 48 h electrocardiographic (ECG) monitoring findings were compared in 27 patients with and 56 patients without giant negative T waves. Patients with giant negative T waves were older at diagnosis (43 +/- 15 versus 32 +/- 14 years, p less than 0.005), had greater ECG voltage (SV1 + RV5 = 57 +/- 20 versus 37 +/- 18 mm, p less than 0.001) and had a more vertical frontal plane axis (38.4 +/- 34 versus 13.4 +/- 45 degrees, p less than 0.05). Left ventricular wall thickness on two-dimensional echocardiography was similar at the mitral valve level (mean 16.5 +/- 4 versus 16.6 +/- 3 cm), but was greater at the papillary muscle level (mean 20.7 +/- 5 versus 17.6 +/- 3 mm, p less than 0.01) and apex (mean 23.3 +/- 5 versus 17.3 +/- 3 mm, p less than 0.001) in patients with giant negative T waves. Fewer patients with giant negative T waves had asymmetric septal hypertrophy (12 [44%] of 27 versus 36 [64%] of 56, p = 0.08) or systolic anterior motion of the mitral valve (4 [14%] of 27 versus 25 [45%] of 56, p less than 0.01), whereas left ventricular end-diastolic (44.1 +/- 6 versus 39.6 +/- 5 mm, p = 0.01) and end-systolic dimensions (27.8 +/- 4 versus 24 +/- 6 mm, p less than 0.05) were greater in this group. Nonsustained ventricular tachycardia was seen on ECG monitoring in 21% of patients in both groups.(ABSTRACT TRUNCATED AT 250 WORDS)