Adjuvant immunotherapy with polyadenylic-polyuridylic acid (PolyA.PolyU) was tested in a randomised trial on 300 patients with operable breast cancer, all of whom were treated by surgery with or without radiotherapy. They were randomly divided into an experimental group of 155 patients who were treated with 30 mg PolyA.PolyU intravenously per week for 6 weeks and a control group of 145 patients who received normal saline intravenously on the same schedule. The mean follow-up time was more than 50 months in both groups. The overall survival was significantly higher in the treated group (p less than or equal to 0.05), in whom the 5-year "relapse-free" surival was also increased. In node-positive patients, treatment increased the relapse-free survival (p less than or equal to 0.03) and overall survival (p less than or equal to 0.07). No side-effects were noted. Thus, immunotherapy with PolyA.PolyU appears to be a simple, non-toxic, and efficient adjuvant treatment in operable breast cancer.