The value of systolic anterior motion of the anterior mitral leaflet as a diagnostic marker for hypertrophic cardiomyopathy has been questioned because of its reported occurrence in other heart diseases. To determine the true specificity of systolic anterior motion for hypertrophic cardiomyopathy, 721 echocardiograms from patients with a wide variety of cardiac diseases were reviewed for its presence or absence under basal conditions. Systolic anterior motion of the anterior mitral leaflet was present in only 22 (3.0%) of the 721 patients, giving a specificity of 97 per cent. It was most common in patients with d-transposition of the great vessels (11 of 51, or 21%). With patients having transposition of the great vessels excluded from the analysis, the prevalence of systolic anterior motion of the anterior mitral leaflet was only 1.6 per cent (specificity 98%). Of note, eight of the 11 patients with systolic anterior motion of the anterior mitral leaflet and diseases other than transposition of the great vessels had disproportionate thickening of the ventricular septum, making it exceedingly rare in a patient population with normal septal-free wall thickness ratios (prevalence 0.4%; specificity 99%). Hence, while systolic anterior motion is not pathognomonic of hypertrophic cardiomyopathy, it was an uncommon finding in a large population of patients with a variety of cardiac diseases; when present in such patients systolic anterior motion of the anterior mitral leaflet is usually associated with disproportionate septal thickening.