The excretion of urinary amino acids obtained from acid hydrolysates were studied by gas chromatographic analysis in 16 normal male subjects and 48 patients with cancer of the prostate. There were no significant differences of the most commonly excreted amimo acids in normal or patient groups or between patients with different stages of the disease because of the large variances of excretion within groups as a whole. However, with few exceptions, the patient group excreted isoleucine whereas isoleucine excretion was not detected in the normal subjects under the same experimental conditions. These findings suggest that measurement of the urinary excretion of isoleucine may be a useful parameter for the early detection of prostatic cancer.