The effect of anabolic steroid therapy and estrogen-progestogen substitution therapy on serum concentration of procollagen type III aminoterminal peptide (PIIINP), a measure of collagen synthesis, in postmenopausal women was studied in two double-blind studies: (1) 39 women allocated to treatment with either 50 mg nandrolone decanoate as an intramuscular depot or placebo injections every third week for 1 year, and (2) 40 women allocated to receive either 2 mg 17 beta-estradiol plus 1 mg norethisterone acetate daily or placebo tablets for 1 year. Serum PIIINP was measured every 3 months during the study. Anabolic steroid therapy resulted in a more than 50% increase (P less than .001) in serum PIIINP at 3 months, which thereafter decayed but remained significantly increased throughout the study period. Serum PIIINP showed the same pattern during estrogen-progestogen therapy, but to a lesser degree. We conclude that anabolic steroids stimulate type III collagen synthesis in postmenopausal women, while estrogen-progestogen therapy may have such an effect, but only to a lesser degree.