PURPOSE: To compare the changes in macular volume (MV) between healthy subjects and patients with diabetic macular edema (DME) after an osmotic load and to determine the glycerol permeability (P(gly)) of the blood-retinal barrier (BRB). METHODS: In this unmasked study, 13 patients with DME and 5 healthy control subjects ingested a glycerol solution (0.57 g/mL) of 3 mL/kg body weight (maximum, 250 mL). Subsequently, the MV determined by the retinal maps provided by the optical coherence tomography (OCT) fast macular thickness protocol was monitored at 12 time points for 180 minutes. A mathematical model of glycerol and osmotic water movements across the BRB was constructed to estimate P(gly). RESULTS: Median MV decreased from 7.30 mm(3) (range, 6.68-7.35) to the maximum median DeltaMV of -0.30 mm(3) (25%-75% quartile: -0.34 to -0.25) in the healthy volunteers and from 7.97 mm(3) (range, 6.85-9.89) to DeltaMV of -0.14 mm(3) (25%-75% quartile: -0.19 to -0.08) in the diabetic group (intergroup difference: P < 0.05). P(gly) was 6.1 x 10(-6) (SE 1.8 x 10(-6)) and 74 x 10(-6) (SE 42 x 10(-6)) cm/s in the healthy and diabetic participants, respectively (P < 0.0001). No rebound phenomenon was observed in either group. CONCLUSIONS: The maximum reduction in MV was doubled in the healthy group compared with the diabetic group, whereas the glycerol permeability was 12 times higher in the diabetic participants. These findings confirm the paradigm of BRB breakdown in DME, but also suggest a novel procedure for the determination of retinal permeability to various agents, which is independent of the vitreous condition (ClinicalTrials.gov number, NCT00333671).