We measured the percentage of slow-twitch (ST) muscle fibers in the lateral portion of the quadriceps femoris muscle in 41 healthy sedentary male controls, 35 active male joggers, and 26 male coronary heart disease (CHD) patients. We then compared these percentages with serum levels of total cholesterol (TC), triglycerides (TG), high density lipoprotein cholesterol (HDL-C) and apolipoprotein A-I (apo A-I) found in these 102 middle-aged men. The percentage of ST muscle fibers in all men correlated positively with serum HDL-C (r = 0.57, P less than 0.001) and with apo A-I (r = 0.60, P less than 0.001) and negatively with triglycerides (r = -0.43, P less than 0.001). The proportion of ST fibers in joggers (65%; 61-69%, 95% confidence interval) was higher (P less than 0.001) than in sedentary controls (48%; 44-52%) or in CHD patients (44%; 39-49%). Moreover, 89% of the joggers had a proportion of ST fibers higher than 50%, whilst in sedentary controls and in CHD patients these values were 46% and 38%, respectively. Positive correlations were found between the percentage of ST fibers and both HDL-C and apo A-I in controls (r = 0.33, P less than 0.05 and r = 0.34, P less than 0.05) and in joggers (r = 0.46, P less than 0.01, and r = 0.40, P less than 0.05), respectively. Negative correlations in controls (r = -0.34, P less than 0.05) and in CHD patients (r = -0.43, P less than 0.05) were also found between the percentage of ST fibers and serum TG.(ABSTRACT TRUNCATED AT 250 WORDS)