Basic human urinary arginine amidase (or esterase, called BHUAE) which is only found in male urine, was measured from normal volunteers between the age of 4 and 70 years using D-valyl-L-leucyl-L-arginine-p-nitroanilide as a substrate. BHUAE increases during early adolescence, between 8 to 17 years of age. Then, BHUAE decreases in the twenties and takes a certain range of value in the mature age group, between the late thirties and fifties.In patients with prostate cancer, a significant increase in BHUAE was demonstrated in comparison with the healthy male group (control) over 55 years old. On the other hand, patients with benign prostatic hypertrophy showed no significant elevation of this enzyme activity. It would appear that the measurement of BHUAE in urine can be used as a marker of prostate cancer in an advanced age group.