The purpose of this study was to determine whether the improvement in muscle performance after treatment of hyperthyroidism is only a result of an increased muscle mass or if it also depends on the improvement of intrinsic contractile function. Nine patients with Graves' disease were evaluated 1) at the time of diagnosis, 2) after 1 wk of monotherapy with propranolol, and 3) after the euthyroid state had been achieved with antithyroid drugs. At each evaluation the patients were submitted, on the dominant side, to anthropometric measurements and to skeletal muscle function tests to determine the maximal static voluntary contraction (MAX) and endurance (END); "endurance" is defined as the time limit at maintaining 30% of MAX. Three movements were tested: hip flexion, ankle dorsiflexion, and handgrip. Body weight changed from 53.4 +/- 3.2 to 58.2 +/- 2.9 kg (P = 0.004) and the sum of skinfold-corrected limb circumferences changed from 90.7 +/- 3.1 to 94.4 +/- 3.1 cm (P = 0.017). MAX and END of all movements increased at the end of the study even if adjusted for the sum of skinfold-corrected limb circumferences: Hip flexion: MAX 20.60 +/- 3.32 to 31.26 +/- 5.07 g.cm-1, END 0.43 +/- 0.18 to 1.18 +/- 0.42 kg.s-1.cm-1. Ankle dorsiflexion: MAX 12.34 +/- 1.97 to 26.88 +/- 2.46 g.cm-1, END 0.97 +/- 0.28 to 2.50 +/- 0.58 kg.s-1.cm-1; Handgrip: MAX: 2.20 +/- 0.23 to 2.9 +/- 0.2 g.cm-1, END 0.13 +/- 0.01 to 0.20 +/- 0.02 kg.s-1.cm-1. In conclusion, improved muscle performance resulting from the treatment of hyperthyroidism is a consequence of an enhanced intrinsic muscle function as well as a greater muscle mass.