Unlabelled: Hypertrophic cardiomyopathy is characterized by a diverse clinical and morphological spectrum. In this report we describe five patients with atypical hypertrophic cardiomyopathy and significantly dilated atria. Diagnostic difficulties are stressed. All patients underwent clinical examination, noninvasive studies including 2-D echocardiogram, cardiac catheterization. In three patients endomyocardial biopsy was taken and examined by light microscopy. All but one patient were men, ranging in age from 28 years to 40 years at initial examination. In all patients disproportion between mildly elevated or, in two cases normal filling pressures and degree of dilatation of both atria was found. Two patients C.G. and D.T. had auscultatory and roentgenographic findings of mitral stenosis. Following echocardiographic and angiocardiographic studies an increase in left and right ventricular thickness associated with dilatation of both atria was found. Endomyocardial biopsy in both cases did not show endocardial thickening or infiltrative changes. In family S, two patients W.S. and Je.S had marginal left ventricular hypertrophy. The presence of left ventricular hypertrophy in patient Ja. S and finding, after family studies, hypertrophic, obstructive cardiomyopathy in patient A.S. enabled establishing the correct diagnosis. Beside varying degree of hypertrophy in family S, mild dilation of the right ventricle and incomplete right bundle branch block were found. In two patients of family S. restrictive cardiomyopathy was found, in patient Ja. S. during cardiac catheterization at initial presentation, in patient W.S. on doppler transmitral flow velocity examination at late follow-up. Paroxysmal atrial fibrillation was the first symptom in four patients, thromboembolic event in one patient. In four patients pacemaker requirement was found. During long-term follow-up (mean 4.8 years) slowly progressive heart failure associated with further dilatation of atria is observed. Mild doses of diuretics are effective in controlling congestive symptoms., Conclusion: in hypertrophic cardiomyopathy significantly dilated atria and clinical signs of mitral stenosis can be present. The presence of myocardial hypertrophy is not necessary to diagnose hypertrophic cardiomyopathy. Familial studies can be helpful in establishing the correct diagnosis.