The therapeutic effect of tiopronin on incipient senile cataracts was evaluated in 150 cases (275 eyes) by a double-blind study using nonactive placebo. A more favorable tendency to visual acuity was noted in the tiopronin than in the placebo group. The slit-lamp microscopic findings showed a statistically significant difference between the tiopronin and the placebo groups. Integrated judgement revealed tiopronin to be significantly effective as compared with the placebo. A side effect of fever with systemic eruption was observed in 1 case in the tiopronin group but disappeared completely when administration of the drug was discontinued.