One hundred and twenty-nine (87%) out of a county population of 150 eligible children with type 1 diabetes mellitus (DM) and 144 age- and sex-matched control children participated in a longitudinal, epidemiological study of the evolution of diabetic microvascular disease. The mean (SD) age of the children with DM was 12.4 (+/- 3.4) years with a mean DM duration of 4.5 (+/- 3.2) years and a mean HbA1c of 11.1 (+/- 2.2)%. Two sets of measurements were made over a period of 18 months. Pupillary adaptation in darkness (PD), as an index of sympathetic dysfunction, was assessed using a portable Polaroid pupillometer. Urinary albumin excretion, as an index of incipient nephropathy, was assessed in children with DM from a fraction of all voidings during two separate 48-h periods and was expressed as urinary albumin/creatinine ratio (A/C). Heart rate variation (HRV), as an index of cardiovascular autonomic dysfunction, was assessed using a heart rate monitor and computer. Blood pressure (BP) was measured using a random zero sphygmomanometer. Reduced PD was found in 14/129 (13.8%) children with DM and in 5.8% of the controls. The diabetic children with reduced PD had longer DM duration, higher HbA1c, raised diastolic BP and higher urinary A/C than those with normal PD. Fifty percent of these children also had increased A/C and impaired HRV, in addition to impaired PD. Another group of 25/129 (19.4%) children with DM presented intermittently raised mean urinary A/C, while five children presented persistently raised A/C in both 48-h urine collections. The characteristics which differentiated the microalbuminuric group from the normoalbuminuric one were older age, longer DM duration, poorer glycemic control, reduced PD, impaired HRV and raised diastolic BP. It seems that diabetic nephropathy (DN) and autonomic neuropathy (DAN) exist in concert in childhood DM. Diabetic children with impaired indices of DN and DAN are older, pubertal and have longer DM duration and higher HbA1c values than the rest of the diabetic population.