We detected limited mobility of small and large joints in 92 (30 per cent) of 309 patients with diabetes who were one to 28 years old. Among patients who had had diabetes for more than 4.5 years (the shortest duration at which microvascular complications were noted), 82 of 169 had joint limitation. Forty-one of these 82 also had microvascular complications, but only 10 of the 87 patients without joint limitation had complications. Life-table analysis indicated an 83 per cent risk for microvascular complications after 16 years of diabetes if joint limitation was present, but only a 25 per cent risk if joint limitation was absent. Consequently, limited joint mobility identifies a population exceptionally at risk for the early development of microvascular complications, and intervention to forestall or prevent these complications can now be focused.