Cases with complicated cataract occurring after vitrectomy and gas-liquid exchange were reported. In a consecutive series of 62 eyes having undertaken the operation, a transient cataract developed in 8 eyes and an irreversible cataract in 28 eyes. The manifestation of the transient cataract is the posterior subcapsular opacity, feathery or placoid vacuolar in clusters in appearance. It was found that if the amount of the residual vitreous at the retrolental space is great enough, the chance of occurrence of complicated cataract is less. That the direct contact of the gas with the lens affects the lens metabolism and nutrition is the chief cause of the development of the cataract. It is suggested that to prevent the development of cataract induced by the gas, besides selection of body posture, quiet stay in bed and proper selection of gas type, the vitreous at retrolental space be resected as little as possible.