Corneal crosslinking (CXL) with riboflavin and UVA irradiation is the standard treatment for preventing progression of keratoconus. Thinning of the cornea during the riboflavin loading phase was reported in previous studies which used riboflavin with dextran. A minimum stromal thickness of 400 [mu]m to prevent endothelial damage is the only critical factor for a safe procedure. Using hypotonic riboflavin or riboflavin with hydroxypropyl methylcellulose (HPMC) may be expected to swell the cornea and keep the corneal thickness at desired levels, however, the gain in corneal thickness was lost within 10 minutes. The authors aimed at looking at the effect of eyelid closure on corneal thickness during riboflavin (RF) induction phase. This was a prospective randomised study including 52 eyes of 48 patients with progressive keratoconus. All patients underwent corneal crosslinking with accelerated protocol. The patients were divided into two groups: group 1 removal of the eyelid speculum during the 20-minute RF (0.1%) + HPMC instillation and group 2 retaining the eyelid speculum in place during the entire CXL procedure. If the cornea was thinner than 400 [mu]m, these patients were excluded. Twenty-six eyes in group 1 and 26 eyes in group 2. There was no statistically significant difference in the mean preoperative pachymetric measurements between the two groups. Corneal thickness significantly increased throughout the procedure in group 1-after 20 minutes of RF instillation (487.46+/-27.25 [mu]m, P<0.0001), at fifth minute (479.69+/-31.26 [mu]m, P=0.009), and at 10th minute (482.19+/-26.52 [mu]m, P=0.004) of UVA irradiation. Corneal thickness reached preoperative levels in group 2-after 20 minutes of RF instillation (456.70+/-34.56 [mu]m, P=0.534), at fifth minute (450.26+/-33.54 [mu]m, P=0.053) and at 10th minute (451.88+/-37.84 [mu]m, P=0.066) of UVA irradiation. No adverse effects were recorded. The authors conclude that closure of eyelids during riboflavin induction further induces corneal swelling. This may offer an advantage to improve safety of the procedure and reduce the need for hypotonic riboflavin in thin corneas. Also this could be cost-effective as less use of riboflavin is needed during lid closure.