Body composition calculated from total body potassium and skeletal muscle potassium were studied in middle-aged obese men and women with normal and impaired glucose tolerance as well as Type II diabetes before and after advice on calorie reduction during twelve months. The subjects were compared with healthy lean men and women. Mean weight loss was 6.6 kg (P< 0.05). Lean body mass and body fat decreased 2.0 kg (P< 0.05) and 4.6 kg (P< 0.05), respectively. Total body potassium decreased by a mean of 146 ± 49 mmol (P< 0.01). Obese men with Type II diabetes and impaired glucose tolerance had lower total body potassium and muscle potassium levels than obese healthy men. After dieting, the obese men and women increased their muscle potassium levels with a mean of 2.8 mmol/100 g fat-free dry weight to 42.6 ± 2.6 mmol/100 g fat-free dry weight (P< 0.05), but they were still below the levels of the lean controls, 44.4 ± 1.3 mmol/100 g fat-free dry weight, (P< 0.01). Increase in skeletal muscle potassium was correlated to decrease in body weight, r = 0.55 (P< 0.01) and to decrease in fasting blood glucose, r = 0.42 (P< 0.05).