The basal energy expenditure of 10 Type 1 (insulin-dependent) C-peptide-negative diabetic patients (2042 +/- 62 kcal/24 h) was found to be significantly higher than the 1774 +/- 52 kcal/24 h predicted from their age, sex and body surface area (p less than 0.01). Intravenous insulin treatment significantly reduced energy expenditure to 1728 +/- 19 kcal/24h (p less than 0.01), which matched predicted values. The observed increase in metabolic rate in uncontrolled diabetic patients is associated with increased protein turnover, increased plasma glucagon, but no significant increase in cortisol, growth hormone or triiodothyronine concentrations in plasma. It may be accounted for by the energy cost of protein synthesis, or gluconeogenesis, or possibly increased sympathetic activity. This increased energy expenditure will contribute to the weight loss seen in Type 1 diabetic patients.