To determine the effects of anabolic steroids on serum lipid and apoprotein levels, 14 white male body builders who self-administered steroids for 2 to 3 months (steroid users) were studied; 10 agreed to screening while they were taking the drugs (ON treatment) and also at about 3 months following their suspension (OFF treatment). Controls consisted of 17 body builders who had never taken steroids (nonusers), and a group of 18 healthy sedentary subjects (controls). During the period of steroid administration, there was a slight reduction in total serum cholesterol, with a marked cholesterol decrease in the high-density lipoprotein (HDL) subfractions HDL2 and HDL3, and a significant reduction in the HDL2 cholesterol/HDL3 cholesterol ratio; the percentage of serum cholesterol transported by low-density lipoproteins (LDL) increased significantly. In addition, a marked apoprotein (apo) A-I reduction in the HDL2 and HDL3 subfractions was observed, as well as an apo A-II decrease that was significant only in the HDL3 subfraction, with an A-I/A-II ratio significantly reduced in both subfractions. Serum apo B was only slightly increased, with a very high B/A-I ratio. Apolipoprotein C-II and E levels showed no modifications, while apo C-III reduced significantly. Lipid and apoprotein values returned to almost normal levels in the OFF treatment period. Findings in the group of nonusers were similar to those in sedentary subjects. These results indicate that anabolic steroids profoundly alter the serum lipid-protein profile, and the changes may be caused in part by the significant differences observed in apoprotein levels.