The pupillary response to light was examined by infrared television-videopupillography in 93 Type 1 (insulin-dependent) diabetic patients (aged 25-42 years, duration of diabetes 0-32 years), and 37 control subjects (aged 26-41 years) with techniques ensuring equality of stimulus and retinal sensitivity, and allowing a detailed computerized calculation of the various parameters of the response. There was no difference in latency time or constriction time between diabetic patients and control subjects. The diabetic patients had a smaller initial pupil size (p = 0.012) and a smaller response amplitude (p less than 0.001) than the control subjects, and these two parameters were correlated to each other (r = 0.49, p less than 0.000001) and inversely correlated to the duration of diabetes (r = 0.26, p = 0.013 and r = 0.29, p = 0.0051, respectively). As a group, the diabetic patients had a relative response amplitude that was similar to that of the control subjects. However, more detailed analysis showed that the diabetic patients with pupil size in the normal range had a small, but significant, reduction in relative response amplitude (p = 0.0021). The maximal velocities of constriction and re-dilatation were reduced in the diabetic patients (p less than 0.001 in either case), but both parameters were intimately correlated to the response amplitude (r = 0.91, p less than 0.000001, and r = 0.79, p less than 0.000001, respectively), and this relationship was identical in the control subjects.(ABSTRACT TRUNCATED AT 250 WORDS)