The influence of age on the response of plasma lipids, body composition, and cardiovascular performance to physical training and detraining was studied in 12 older and 12 young adult male cyclists. The athletes were first examined at the peak of their seasonal preparation and then again 2 months after its suspension. Sedentary males matched for age, weight, and height comprised the respective control groups. During training, body fat mass (BFM) was significantly lower and maximum oxygen consumption (VO2max) higher in both groups of cyclists as compared with controls. No differences in serum total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), apolipoprotein (apo) B, apo A-II, and fibrinogen were found. During the same phase, triglycerides (TG) and the LDL-C to high-density lipoprotein cholesterol (HDL-C) ratio were significantly lower and apo A-I, HDL-C, HDL3-C, and the apo A-I/apo B ratio were significantly higher in the athletes than in their corresponding sedentary controls. After physical deconditioning, BFM increased and VO2max decreased significantly in both groups of athletes. TG, very-low-density lipoprotein cholesterol (VLDL-C), and fibrinogen increased in young athletes while the LDL-C/HDL-C ratio increased, and apo A-I, HDL-C, HDL2-C, and HDL3-C decreased significantly in both young and older athletes. Thus, an aerobic training program induced an antiatherogenic lipoprotein profile and beneficial modifications in body composition and aerobic power in both older and younger subjects; a 2-month interruption in the program changed these parameters unfavorably in both groups. Age does not seem to influence significantly the plasma lipid response to physical deconditioning.