SummaryobjectiveWe aimed to study the effect of biosynthetlc growth hormone (GH) replacement In growth hormone deficient adults.designWe performed a double‐blind placebo‐controlledcross‐over study of 6 months biosynthetic GH, replacement and 6 months placebo separated by a 1‐month'swashout period.patientsFourteen growth hormone deficient adults were studied.measurementsWe measured total body weight, percentage fat mass, lean body mass, muscle volume, exercise capacity, maximum oxygen consumption, muscle strength, bone mineral content, a number of biochemical parameters, IGF‐I, GH antibodies andpsychological well‐being.resultsTotal body weight remained unchanged, but lean body mass increased (before GH mean ± SEM 49·8±5·5, after 53·4±5·6 kg; placebo before 51·2±5·4, after 50·4±5·1 kg;P<0·05 and fat mass decreased (before GH 21·5±4·1, after 19·3±4·3, placebo before 19·3±4·0, after 22·5±4·5 kg;p<0·05). Thigh muscle volume increased: (before GH 94·1±7·7, after 99·5±8·4 ml; placebo before 99·3±8·6, after 95·4±7·8 ml/0·8 mm computerized tomographic slice;P<0·05). Exercise capacity increased (before GH 174·15, after 199·18·9 watts; placebo before 162·5±2·3, after 154·19·8 watts;P<0·05), as did maximum oxygen consumption (before GH 1·93±0·2, after 2·17±0·2 I/m; placebo before 1·92±0·3, after 1·98±0·2 I/m;P<0·05). There was no change in quadriceps muscle strength. Alkaline phosphatase increased (before GH 87·5 (32–158), after 106·0 (49–179) U/l; placebo 98·5 (50–145), after 72·0 (40–111) U/l;P<0·05) without a change in the spinal bone density. IGF‐I increased (before GH 62 (36–97), after 216 (62–362)μg/l; placebo before 59 (52–112), after 60·5 (38–94)μg/l;P<0·05). Carbohydrate tolerance remained unchanged as did fasting lipids, serum sodium, potassium, urea, calcium, phosphate and liver transaminases. Psychological well‐being remained unchanged. No growth hormone antibodies were detected before or after GH treatment.conclusionsGH alters the body composition of growthhormone deficient adults and leads to improved exercisecapacity; alkaline phosphatase act