Alterations in plasma branched-chain amino acids (valine, isoleucine and leucine) and alanine have been described in patients with insulin-dependent diabetes mellitus who have poor metabolic control. To assess the relevance of these abnormalities as indices of metabolic control, we sequentially evaluated plasma amino acids in 14 poorly controlled diabetics (seven Type 1 (insulin-dependent) and seven Type 2 (non-insulin-dependent) patients) until good control was achieved. The sum of branched-chain amino acids in both groups of uncontrolled diabetic patients was significantly increased compared with the values for the same subjects in good metabolic control. No statistically significant differences were present between ketotic and non-ketotic uncontrolled patients. The amelioration of the diabetic state with either insulin treatment or oral hypoglycaemic agents, reduced progressively branched-chain amino acids. The sum of valine, isoleucine and leucine strictly correlated with daily urinary glucose (r = 0.73), but less well with fasting blood glucose (r = 0.43), non-esterified fatty acids (r = 0.46) and glycosylated haemoglobin (r = 0.38). Alanine did not show any statistically significant differences at various stages of diabetic control. Branched-chain amino acids, but not alanine, may be used as indices of short-term diabetic control.