Diabetic patients routinely have their pupils dilated for fundoscopy as part of the annual review. To assess the ability of diabetic patients to drive after pupillary dilatation we studied 61 diabetic patients (18 IDDM, 43 NIDDM), mean age 54.98 years, before and 1 h after pupillary dilatation with 1% tropicamide. Binocular visual acuity (BVA) and contrast sensitivity were checked without glare, with glare, and with glare and sunglasses. Glare was introduced using a 60 W bulb in all 61 patients and with a 500 W bulb in 37 of these patients. Prior to dilatation all 61 patients had a BVA of 6/9 or better. A significant reduction in BVA was found post-dilatation (p = 0.005) and 4 out of the 61 patients (6.56%) had a post-dilatation BVA of less than 6/9. The 60 W glare source caused a significant reduction in BVA pre-dilatation (p < 0.05), but not the 500 W glare. With glare, post-dilatation BVA reduced further, resulting in 6 and 7 patients having a BVA of less than 6/9 with the 60 W and 500 W glare source, respectively. The addition of sunglasses with glare did not improve the BVA. No patient with a BVA of 6/5 pre-dilatation reduced to less than 6/9 post-dilatation. No significant change in contrast sensitivity was found in any of the test conditions. We conclude that patients who meet the visual legal requirements to drive (BVA < or = 6/9) prior to dilatation may not fulfil them post-dilatation. This has important clinical implications and the time course of the phenomenon requires exploration. Meanwhile, patients need to be warned not to drive after pupillary dilatation when they attend for annual fundoscopic examination, certainly for at least two hours.