The long-term outlook of patients with brittle insulin-dependent diabetes is uncertain. We assessed the outcome of a group of young female patients with diabetes and recurrent ketoacidosis originally investigated in 1979-85 and reassessed after a mean of 10.5 (SD 1.4) years. 7 of the 33 patients could not be traced. 5 (19%) of the remaining 26 had died. Causes of death were not certain, but were probably ketoacidosis (2), hypoglycaemia (2), and renal failure (1). Of the 21 survivors, only 2 (10%) were still considered to have brittle diabetes. Diabetic complications were common (67%), and were more frequent than in a matched control group of stable patients with diabetes (25%). Brittle diabetic patients also had lower quality-of-life scores, more frequent psychosocial disruptions, and were on higher insulin doses (77  vs 47  U per day, p = 0.007) than controls. Pregnancy complications had occurred in 13 of 28 (46%) pregnancies in severely unstable patients compared with 2 of 27 (7%) in stable controls. Patients with brittle diabetes have a tendency to become more stable with time, but have a higher risk of death, more microvascular and pregnancy complications, and a poorer quality of life.