To investigate whether bizarre myocardial hypertrophy with disorganization (BMHD) is characteristic of hypertrophic cardiomyopathy (HCM), the histopathology of the biopsied left ventricular myocardium in 18 patients with essential hypertension (HT) and 14 patients with HCM was studied. A "biopsy score" was devised for a more quantitative evaluation of the BMHD and a comparative study on the biopsy score of the left ventricular biopsied specimen was also performed. The patients with HT were judged to be in stages I or II of the WHO criteria and had a history of hypertension of more than 5 years. The BMHD was defined as myocardial cells showing hypertrophy, disorganization, and bizarre nuclei. "Disorganization" of myocardial cells was distinguished both by the terminology and histopathological characteristics from "disarrangement" of myocardial cells. The biopsy score employed four factors and was determined according to the following formula: Biopsy score = hypertrophy of myocardial cells + (disorganization of myocardial cells) x 2 + bizarre nuclei + whorling of muscle bundles. Both the hypertrophy and the disorganization of myocardial cells were regarded as essential conditions indicating the presence of BMHD. The BMHD was found in 2 of 18 patients with HT (11%) and in 10 of 14 patients with HCM (71%) in the left ventricular biopsied specimens (P less than 0.005). However, "disarrangement" of myocardial cells was found in 13 of 18 HT patients (72%) and in 10 of 14 HCM patients (71%) in the left ventricular biopsied specimens, showing no difference between the two groups. The biopsy score in HCM patients was larger than that found in HT patients.(ABSTRACT TRUNCATED AT 250 WORDS)