The primary aim of these analyses was to explain the contribution of changes in sex steroid hormones and fat patterning to changes in plasma concentrations of high (HDL-C) and low (LDL-C) density lipoprotein cholesterol, triglycerides (TG), and apolipoproteins (apo) Al, All, and B occurring during puberty in males. Sex steroid hormones assayed included estradiol (E2) and free testosterone (T). The following hypotheses were tested: (1) increasing free T predicts/leads to decreases in HDL-C and increases in LDL-C, apo B, and the LDL-C/HDL-C ratio in adolescent males; (2) increasing E2 predicts decreases in apo B, LDL-C and the LDL-C/HDL- C ratio, but the resultant effects will vary with adiposity and fat patterning; (3) rapid weight gain predicts increased central adiposity, defined as the ratio of truncal skinfolds to total skinfolds, and with greater decreases in HDL-C and increases in triglycerides, apo B, LDL-C and the LDL- C/HDL-C ratio. Rapid weight gain predicts increased E2, but the atherogenic effects of increased central adiposity on lipids are greater than the anti- atherogenic effects of E2. These analyses provided a better understanding of metabolic factors underlying obesity-hormone-lipoprotein relationships.