Previous clinical studies in patients with hypertrophic cardiomyopathies predominantly comprised patients with typical subaortic HOCM and in many other studies patients with 'hypertrophic' cardiomyopathies were investigated; i.e. no distinction was made between patients with the typical obstructive, the atypical obstructive and the non-obstructive forms of hypertrophic cardiomyopathy. In addition, in the more differentiated studies, the differential diagnosis between HOCM and HNCM was based mainly on the presence and absence of the subaortic pressure gradient. This is also true for the clinical characterization of both types of hypertrophic cardiomyopathy. Finally, diagnostic descriptions of atypical (mid-ventricular) HOCM exist only in the form of case reports, i.e. they are not based on observations in a large number of patients. Therefore a comparative study in 353 patients with hypertrophic cardiomyopathies was designed to characterize the clinical picture of patients with HNCM and with atypical HOCM in relation to typical HOCM with special reference to the differential diagnosis. The study revealed a profoundly different diagnostic profile of the three subsets of hypertrophic cardiomyopathies. The distinction between the three subsets may serve as a more differentiated basis for clinical studies related to diagnosis, therapeutic approaches, long-term follow-up observations and prognostic evaluation of patients with hypertrophic cardiomyopathies.