We conducted a randomized controlled study of streptococcal preparation OK-432 on 120 newly identified cases of laryngeal squamous cell carcinoma who were registered at 10 participating institutions between November 1984 and October 1989. The patients were divided into two groups: those in early stages (stage I or II) and those in advanced stages (stage III or IV); these groups were further subdivided into an immunotherapy group (receiving OK-432) and a control group (who did not receive OK-432). The usefulness of OK-432 was studied using the sealed envelope method. The basic therapy for all cases was radiotherapy and, when required, surgery. As adjuvant therapy, 5Fu or derivatives were administered to all cases from the beginning of the treatment period to one year after the basic therapy, with the exception of cases in whom side effects were serious enough to contraindicate use of the drug. The target administration period was 5 years. Of the initial 120 cases, 11 cases were disqualified (3 cases of double cancer and 8 of incomplete primary therapy) and the remaining 109 were used for evaluation. The 5-year survival rate and the 5-year recurrence-free rate were 76% and 84%, respectively, in the immunized groups (both the early and advanced groups), whereas the same rates for the control groups were 78% and 75%. There was a tendency for the immunized groups to enjoy a slightly longer recurrence-free period. Over a 24-month observation period the immunized group always had higher levels of peripheral leukocytes and peripheral lymphocytes; this difference was significant for the first 21 months. Inhibition of bone marrow function is sometimes observed with radiotherapy. It is hoped that, if this inhibition can be mitigated, it will be possible to assist the compromised immune system and maintain a certain level of immune performance which will prevent recurrence and improve survival rate. In the present study we observed a tendency of the lower recurrence rate in the immunized group, and we hypothesize that OK-432 is effective in extending the recurrence-free period.