The role of endomyocardial biopsy (EMB) in assessing of idiopathic dilated cardiomyopathy (IDCM) is a well-recognised one. On the contrary, the value of correlating histological features, such as myocellular hypertrophy, with functional evaluation and outcome is still controversial. It was the purpose of the present study to appraise the correlation with hemodynamic data and the predictive role of histological features in 32 consecutive patients affected by IDCM who underwent left ventricular EMB between January 1984 and December 1986. Light microscopy findings were graded by means of a semiquantitative score system. In comparison with the 19 patients with mild myocellular hypertrophy, the 13 patients with marked hypertrophy showed significantly lower right ventricular end-diastolic pressure (10.4 +/- 5.8 vs 6.6 +/- 3.6 mmHg, p less than 0.05) and left ventricular end-diastolic pressure (26.9 +/- 9.0 vs 16.5 +/- 8.8 mmHg, p less than 0.01). On the contrary, different degrees of interstitial fibrosis, as well as of the other morphologic findings, could not identify patients with distinct hemodynamic patterns. However, there was a direct correlation between the amount of myocellular hypertrophy and interstitial fibrosis (p less than 0.01). During a mean follow-up period of 32 +/- 11 months (range: 12-48 months), 6 patients died and 3 further patients underwent heart transplantation. Univariate analysis of histological features (log-rank test) showed a mild degree of hypertrophy alone to be significantly related to a poor outcome (p = 0.02).(ABSTRACT TRUNCATED AT 250 WORDS)