Echocardiographic and electrocardiographic findings in 74 adults with hypertrophic cardiomyopathy (HCM) were analyzed to identify the pattern of myocardial hypertrophy as a possible determinant of abnormal Q waves. The pattern of septal hypertrophy along the left ventricular long axis was divided into 3 types based on the site of maximum septal hypertrophy: basal, diffuse and apical types. Abnormal Q waves defined by the revised Minnesota Codes (either I-I, I-II or I-III) were noted in 31 cases (42%). The total incidence of abnormal Q waves in the basal type (15/26, 58%) and in the diffuse type (12/22, 55%) was significantly higher (p less than 0.001 and p less than 0.01, respectively) than that in the apical type (4/26, 15%). The abnormal Q waves defined by the strict criteria of Code I-I were significantly more prevalent (p less than 0.05) in the basal type than in the diffuse type, although there was no significant difference in the total incidence of abnormal Q waves between these 2 groups. Thirty-six patients with an extension of hypertrophy to the right ventricle (RVH) had a significantly higher incidence of abnormal Q waves than 22 patients without RVH (56% vs 27%, p less than 0.05). Furthermore, close relationships of RVH to the location of abnormal Q waves were documented. In conclusion, the abnormal Q waves in HCM may be related to the pattern of septal hypertrophy along the left ventricular long axis and to RVH.