The clinical courses of 10 eyes of five diabetic patients who exhibited bilateral transient hyperopia (maximum: 1:1-4.9 dioptres, spherical equivalent) after initiation of strict control of diabetes with or without insulin are reported. The hyperopia occurred within a few days after abrupt decrease in plasma glucose, progressed to maximum at days 7-14, and regressed gradually over 1 month thereafter. Transient cycloplegia had no effect on refractive error. During hyperopia, there were no significant changes in axial length or corneal curvature. However, thickened lens, decreased anterior chamber depth, and transient cataract were observed to significant degrees. It is suggested that the transient hyperopia, with lens swelling and opacity, was caused by decreased lens refractive index following water influx.