To determine the effect of weight loss on serum high-density lipoprotein cholesterol (HDL-C), we measured serum HDL-C as well as total cholesterol and triglycerides in 65 subjects 56 women and 9 men, mean age 41.1 +/- 1.5 (+/- SEM yr) before and after a weight reduction program. At entry into the program there was a significant correlation between HDL-C and several indices of overweight--relative weight, body mass index and sum of skinfold thickness. For all subjects, despite a significant weight loss of 4.5 kg or 5.8% of initial body weight and significant decrease in sum of skinfold thickness, there was no significant increase in HDL-C or correlation between changes in HDL-C and change in body weight or skinfold thickness. For women, but not men, a weak negative correlation between change in HDL-C and change in weight or percentage change in weight was observed. However, in the subset of women who were current cigarette smokers a significant (p less than 0.01) correlation was observed between change in HDL-C and change in weight (r = -0.876) and percentage change in weight (r = -0.881). Thus a modest weight loss is not usually associated with a significant increase in serum HDL-C concentration except in cigarette smoking women.