A 40–year–old man developed dyspepsia, watery diarrhea, and weight loss. A clinical diagnosis of hyperthyroidism due to Graves’ disease was confirmed [free thyroxine, 87 pmol/L (normal range, 8.6–27 pmol/L)]. Bile acid malabsorption was demonstrated by a low 23– selena–25–homocholyltauric acid retention of 7.1% (normal, >15%). Antithyroid treatment (carbimazole and propranolol) was instituted, and his diarrhea subsided with the control of hyperthyroidism (free thyroxine, 15 pmol/L) along with an improvement of bile acid absorption (Se–HCAT retention, 14.7%). This case very strongly suggests the existence of bile acid malabsorption in hyperthyroidism. The temporal association suggests that the diarrhea may have been due at least in part to bile acid malabsorption, raising the possibility that the latter may be an etiological factor in thyrotoxic diarrhea. We believe this is the first such report.