The effects of growth hormone treatment of adults with adult-onset pituitary insufficiency on lipoproteins and apolipoproteins were investigated. Nine patients, one women and eight men (age range, 34-58 years), who had been treated for pituitary tumors were studied. They had complete pituitary insufficiency with a duration of at least 1 year. All patients received replacement therapy with thyroid hormones, glucocorticoids, and gonadal steroids. The study had a double-blind, placebo-controlled, crossover design for active treatment with recombinant human growth hormone (0.25-0.5 units/kg per week s.c given each evening) for 6 months. Fasting serum levels of cholesterol; triglycerides; high density lipoprotein and low density lipoprotein cholesterol; apolipoproteins A-I, B, and E; and lipoprotein (a) were measured before and after 6 and 26 weeks of treatment Lipoprotein (a) concentrations increased markedly during treatment and were about twice as high compared with pretreatment levels. Serum cholesterol and low density lipoprotein cholesterol concentrations were decreased after 6 weeks of treatment, but levels had returned to pretreatment levels after 26 weeks. High density lipoprotein cholesterol concentrations increased during treatment and were significantly higher than pretreatment levels after 26 weeks of treatment Serum trigiyceride concentrations did not change significantly, but in two patients with marked hypertriglyceridemia, growth hormone treatment resulted in a marked decrease. Serum concentrations of apolipoproteins A-I, B, and E did not change significantly, but changes in apolipoprotein A-I and B concentrations were in parallel to those observed for high density lipoprotein cholesterol and low density lipoprotein cholesterol, respectively. These results suggest that growth hormone is a major regulator of lipoprotein metabolism and also demonstrate that lipoprotein (a) concentrations are regulated by growth hormone. (Arteriosclerosis and Thrombosis 1993;13:296-301) KEY WORDS • lipoproteins • growth hormone • hypopituitarism F ew human studies have been performed on the effects of growth hormone (GH) on the different lipoprotein fractions and apolipoprotein levels. Moreover, studies on the effects of GH on the regulation of serum lipid levels have given conflicting results. However, both GH excess, as in acromegaly, 1 and deficiency 2 result in an increased risk of death due to cardiovascular disorders, which might indicate a role for GH in the control of lipoprotein metabolism in humans.Several recent publications have reviewed the genetics, biochemistry, and possible role in arteriosclerosis and thrombogenesis of lipoprotein (a) (Lp[a]). 3~6 The plasma concentration of Lp(a) is mainly genetically From the Department of Physiology (S.E., J.O.), the