The tear turnover was determined by fluorophotometry in 25 insulin-dependent diabetes mellitus (IDDM) patients without retinopathy and 29 IDDM patients with (pre-)proliferative retinopathy. The results were compared with those in 34 healthy controls, to investigate the lacrimal gland function in diabetic patients. The tear turnover was calculated from the decay of the relative tear fluorescein concentration values measured after instillation of one microliters of fluorescein. The tear turnover values in both patient groups did not correlate significantly with age or diabetes duration (linear correlation coefficients: r less than 0.3). The tear turnover values in patients both without retinoplathy and with (pre-)proliferative retinopathy did not differ significantly from those in healthy controls (mean +/- SD in %/min: 13.7 +/- 4.5, 14.7 +/- 5.8 and 15.5 +/- 5.1, respectively; P greater than 0.16). The tear turnover was significantly decreased in eyes having a BUT shorter than 10 seconds compared with eyes having a BUT longer than 10 seconds (P less than 0.05). The tear turnover values correlated significantly with the HbA1c and Schirmer-test values in patients with (pre-)proliferative retinopathy (r = 0.7 and r = 0.4, respectively; P less than 0.02) and with the blood glucose values in patients without retinopathy (r = 0.41, P = 0.04). Since the tear turnover was not significantly decreased in IDDM patients in comparison with healthy controls the corneal disorders which are more frequently seen in these patients than in a healthy population may not be attributed to a decrease in tear production.