An in vitro technique for isolating prostatic cells from blood has been applied to the study of circulating prostatic cells during a transurethral resection of the prostate. Our findings indicate that prostatic cells that are added to the blood in vitro can be isolated by the technique described and that cells resembling prostatic cells can be isolated from the blood of the patient undergoing transurethral resection of the prostate. Furthermore, the results of our study suggest that (i) in selected patients, needle biopsy of the prostate should be carried out prior to transurethral resection of the prostate and that (ii) patients who have multifocal adenocarcinoma diagnosed at the time of transurethral resection of the prostate may not be suitable candidates for future prostatectomy. © 1979.