Pupillary adaptation to darkness was studied in 63 children and adolescents with Type 1 diabetes using a simple portable pupillometer. Results were compared with those in a group of age-related non-diabetic children and expressed as the ratio of the pupil diameter to the iris diameter (pupil diameter %). In the diabetic patients the pupil diameter % was 61.1 +/- 5.8 (44.4-71.9) % compared with 64.2 +/- 4.1 (53.2-72.6) % in the control subjects (p less than 0.001). Abnormal pupillary adaptation to darkness was found more commonly than abnormal heart rate variation in response to a variety of stimuli in the diabetic patients. Pupillary adaptation to darkness may be useful as an indicator of subclinical autonomic neuropathy in diabetic children.