OBJECTIVE Growth hormone deficiency in adults with hypopituitarism has previously received little attention. Recent data, however, suggest that GH deficiency might be essential for the long-term prognosis of these patients. Earlier studies have documented that GH regulates body composition; in this, body composition in adult patients with hypopituitarism including GH deficiency was studied. DESIGN A follow-up study of patients with hypopituitarism on routine replacement therapy with L-thyroxine, cortisone acetate and sex steroids. PATIENTS One hundred and six patients (69 males, mean age 52.5 years and 37 females, mean age 53.4 years) diagnosed as having growth hormone deficiency on the basis of low IGF-I concentration or a maximum GH-response less than 5 mU/l after an insulin/glucagon tolerance test. MEASUREMENTS Body composition was estimated from body weight, total body water and total body potassium and the results were compared with values predicted from height, weight, age and sex, using data from a large number of healthy subjects. RESULTS The total body water was significantly lower than that predicted from the observed body weight (P < 0.001), as was the extracellular water (P < 0.001) and the extracellular/intracellular water quotient (P < 0.001). On average, the body cell mass was similar to the predicted value, but the observed/predicted body cell mass ratio correlated positively with age at follow-up. The body cell mass was lower than predicted in subjects below the age of 50 years (P < 0.01). The body fat was higher than predicted (P < 0.001); the increases was also noted in lean subjects. The observed body weight in male subjects was 7.5 kg higher (P < 0.001) than that predicted from healthy subjects of the same body height, a difference explained by an average increase of 6.6 kg in the body fat (P < 0.001) and 1.6 kg in the body cell mass, with a simultaneous reduction of 0.7 kg in the extracellular water (NS). Male patients suffering from untreated androgen deficiency had lower body cell mass than those on androgen treatment. Female subjects weighed 3.6 kg (NS) more on average than healthy women, a difference explained by an increase in the body fat of 6.0 kg (P < 0.001) with a simultaneous decrease of 2.4 kg in the extracellular water (P < 0.001). The body cell mass was similar to that seen in the controls. CONCLUSIONS Adult patients with growth hormone deficiency have an increased body weight compared to normals of the same age, sex and height, due to an increment of the body fat with a simultaneous reduction in the total body water.