The cytotoxic activity of NK cells in peripheral blood was studied in 15 patients with ovarian cancer Stage III and IV before and after surgery and ten-week intramuscular therapy with thiotepa. An evident decrease in NK activity was found 24 hours after surgery, with a slow return toward normal values within 7-9 days. No differences between patients with explorative laparotomy and with extirpation of the tumor were observed. The mechanism of the decreased NK cytotoxicity remains unknown but there was a concomitant reduction in the number of blood mononuclears. A second drop in NK activity occurred after chemotherapy. It was most likely due to the drug bone marrow depression. No evident correlation between the reduced NK activity and advancement of the disease could be found.