Experiences resulting from Emcyt therapy in patients with both newly diagnosed and hormone refractory advanced prostate cancer, as well as on adjuvant to surgery or radiotherapy in earlier disease are presented. Data from trials of the National Prostatic Cancer Project (NPCP) and a series from Roswell Park Memorial Institute (RPMI) were divided into short-term (up to twenty weeks and up to fifty-two weeks in adjuvant trials) and long-term therapy. Baseline disease and patient characteristics and toxicities encountered in these two treatment-duration groups were compared. Patients in a more favorable health or disease status and/or responded to therapy were more frequently in the long-term group. Patients in the long-term group tended to have higher over-all incidences of toxicity; and although many had occurrences begin as early as those in the short-term group, they were able to tolerate the therapy for relatively long periods. The agent is thus both effective and can be given safety for long periods of time.