The authors report the preliminary data of a study on the proliferative activity in urologic cancers. They use the monoclonal antibody KI-67 which can find out a nuclear antigen present only in cells in proliferative phase. They describe the methodology and report the results concerning 12 cancers of the kidney, 17 of the bladder, 17 of the prostate and 2 of the testicle. The most interesting results were obtained in the study of bladder cancer where they could pick out, in the context of the cancers with grading G2, a sub-group with proliferative characteristics similar to those of cancers G3 and vice-versa. They also obtained important results in the study of prostate cancer in hormonal therapy, as they could demonstrate the disappearance of the proliferative activity in patients responding clinically to the treatment.