45 patients (39 men, six women), mean 41 (19-63) years of age with clinical, angiographic and morphologic diagnosis of dilated cardiomyopathy were evaluated in respect of three morphologic classes. Two groups of patients without signs of previous myocarditis were formed, with one-to-two mitochondria per two sarcomeres (group Ia, n = 19), or with more than two mitochondria per two sarcomeres, respectively (group Ib, n = 14); and one group with signs of previous myocarditis (group II, n = 12). The mean relative mitochondrial volume fraction in relation to myofibril volume fraction was significantly lower in group Ia (33 +/- 4/67 +/- 4%) compared to group Ib (39 +/- 5/61 +/- 5%) (p less than 0.01). Mean values of group II (36 +/- 6/64 +/- 6%) were in between the two other groups. Left ventricular enddiastolic pressure (18 +/- 11, 18 +/- 8, 16 +/- 10 mm Hg), pulmonary vascular resistance (473 +/- 414, 406 +/- 205, 458 +/- 495 dyn x s x cm-5), ejection fraction (36 +/- 21, 32 +/- 16, 28 +/- 16%), endsystolic volume index (131 +/- 82, 127 +/- 66, 132 +/- 60 ml/m2), enddiastolic volume index (187 +/- 81, 176 +/- 62, 181 +/- 63 ml/m2), dp/dt max (1951 +/- 875, 1737 +/- 575, 1741 +/- 478 mmHg x s-1) and mean VCF (0.76 +/- 0.58, 0.44 +/- 0.32, 0.54 +/- 0.39 s-1) showed no significant differences between the three groups. Follow-up of the patients in the three groups to median 19, 22, 24 months, respectively, after biopsy, showed an improvement of the clinical findings, especially concerning the groups with one-to-two mitochondria only and with signs of previous myocarditis, but no difference in survival within the three groups. For the individual case our morphologic parameters seem to be without predictive value.