PURPOSE: To investigate 24-hour variation in retinal thickness in patients with diabetic macular edema (DME) using optical coherence tomography (OCT).METHODS: Fifty-three eyes of 53 diabetic patients with clinically significant macular edema and central subfield thickness (CST) >225 µm, 36 eyes of 36 healthy individuals (normal controls), and 22 eyes of 22 diabetic patients without macular pathology (diabetic controls) underwent 5 OCT measurements at 7 am, 10 am, 3 pm, 8 pm, and 1 am. Visual acuity, blood pressure, blood glucose, and body temperature were measured as well.RESULTS: The CST (p<0.0005), total macular volume (p<0.0005), and visual acuity (p<0.0005) showed significant variation in patients. The CST (450 µm at 7 am) reached a minimum at 3 pm (absolute change of -49 µm, relative change of -17%) before increasing again. Thickening changes were higher in more thickened retinas (p<0.0005, p=0.024, absolute and relative change, respectively). Visual acuity was worse in the morning (0.38 logMAR) and improved to a maximum at 8 pm (0.30 logMAR) (p<0.0005). Blood pressure, blood glucose, and body temperature did not vary over time.CONCLUSIONS: The 24-hour variation of retinal thickness is observed in a large proportion of patients with DME, with a decrease from morning to afternoon. Time of examination should be taken into account when managing such patients.