Adult growth hormone (GH) deficiency results mainly from pituitary or peri-pituitary disease and/or its treatment and is frequently accompanied by other anterior pituitary hormone deficiencies. GH deficiency (GHD) results in a number of psychological and physical symptoms and signs which in combination constitute the adult 'GHD syndrome'. The psychological symptoms include decreased energy levels, social isolation, and lack of positive well being, depressed mood and increase in anxiety. The physical symptoms and signs include abnormal body composition with reduced lean body mass, increased central adiposity, and decreased extracellular fluid volume, decreased bone mineral density with an increased risk of fracture, reduced muscle strength, reduced exercise capacity, increased LDL cholesterol and reduced insulin sensitivity. Hypopituitarism and GHD are associated with an increased standardised mortality ratio. The diagnosis of GHD is confirmed by the insulin tolerance test or alternative stimulation test in the presence of structural pituitary disease and/or additional pituitary hormone deficiencies. Replacement with synthetic growth hormone by once daily subcutaneous injection can reverse many of the symptoms and signs of growth hormone deficiency, but the long-term effects are yet to be established. Whether or not all patients with GHD should receive GH replacement remains a matter for debate: a selective approach to therapy based on psychological well being and quality of life has been adopted in many centres.