Endurance fitness has been measured objectively (physical work capacity at pulse rate of 170/min, W 170) in a group of middle-aged executives, and related to a number of other physical characteristics and aspects of coronary risk status: FEV 1 blood pressure, adiposity, smoking habit, alcohol consumption, plasma levels of total and non-high density lipoprotein cholesterol, triglyceride and high density lipoprotein cholesterol (HDL-C). The primary question was whether HDL-C levels could be shown to be related to endurance fitness levels over the range encountered in a fairly homogeneous population and hence whether there could be value in terms of lipid coronary risk status in encouraging a moderate increase in physical activity. HDL-C levels were significantly related to W 170. Fitness also separated the subjects in terms of adiposity, but not in terms of the other variables studied. Even though the trend was toward an index of physical activity being able to separate the subjects in terms of HDL-C, this was not as clear-cut as the division in terms of endurance fitness. Alcohol and smoking were associated with higher triglyceride levels, but not with HDL-C. The variables mid-abdominal skinfold thickness, triglyceride, non-HDL-C and endurance fitness accounted for 53% of the variation in HDL-C levels in this population. Alterations in the levels of these probably related variables might be expected to have appreciable effects on levels of HDL-C.