We investigated 49 men with Peyronie's disease in a prospectively randomized placebo controlled double-blind study, comparing the effects on plaque size and penile deformity of intralesional purified clostridial collagenase and saline placebo. For the group as a whole, treatment out-performed placebo (p <0.007). When patients were analyzed with respect to disease severity, those with lesser deformity responded more favorably to treatment. The absolute angular change in patients responding to treatment was small. No significant side effects were noted within a 3-month followup.