Insulin resistance in acromegaly causes glucose intolerance and diabetes, but it is unknown whether it involves protein metabolism, since both insulin and growth hormone promote protein accretion. The effects of acromegaly and of its surgical cure on the insulin sensitivity of glucose and amino acid/protein metabolism were evaluated by infusing [6,6-²H₂]glucose, [1-¹³C]leucine, and [2-¹⁵N]glutamine during a euglycemic insulin (1 mU · kg¹ · min¹) clamp in 12 acromegalic patients, six studied again 6 mo after successful adenomectomy, and eight healthy controls. Acromegalic patients, compared with postsurgical and control subjects, had higher postabsorptive glucose concentration (5.5 ± 0.3 vs. 4.9 ± 0.2 µmol/l, P < 0.05, and 5.1 ± 0.1 µmol/l) and flux (2.7 ± 0.1 vs. 2.0 ± 0.2 µmol · kg¹ · min¹, P < 0.01, and 2.2 ± 0.1 µmol · kg¹ · min¹, P < 0.05) and reduced insulin-stimulated glucose disposal (+15 ± 9 vs. +151 ± 18%, P < 0.01, and 219 ± 58%, P < 0.001 from basal). Postabsorptive leucine metabolism was similar among groups. In acromegalic and postsurgical subjects, insulin suppressed less than in controls the endogenous leucine flux (9 ± 1 and 12 ± 2 vs. 18 ± 2%, P < 0.001 and P < 0.05), the nonoxidative leucine disposal (4 ± 3 and 1 ± 3 vs. 18 ± 2%, P < 0.01 and P < 0.05), respectively, indexes of proteolysis and protein synthesis, and leucine oxidation (17 ± 6% in postsurgical patients vs. 26 ± 6% in controls, P < 0.05). Within 6 mo, surgery reverses insulin resistance for glucose but not for protein metabolism. After adenomectomy, more leucine is oxidized during hyperinsulinemia.