One hundred seventy-one patients were studied in order to evaluate the clinical efficacy of the transfer factor (TF) for primary resected lung cancers under a randomized controlled trial. Eligible cases for evaluation were randomly chosen at 75 and 74 patients in TF and control groups, respectively. The same long-term intermittent adjuvant chemotherapy was administered to two groups as a standard therapy. The distribution of clinical features in both groups was very similar. The overall survival rates of the TF group at 2 and 4 years postoperatively were 69% and 53%, respectively, which was about 15% better than the control group, but this difference could not yet be considered statistically significant. The survival of the TF group was significantly better than that of the control group in patients with Stages I + II or curative resection (P less than 0.05 by Cox-Mantel test); however, there was no significant difference in patients with Stages III + IV, or noncurative resection. The recurrence rate of pulmonary and mediastinal regions was less in the TF group. In conclusion, TF seems to suppress postoperative recurrence and appears to be beneficial for primary resected lung cancer patients, especially at early stages, as postoperative adjuvant immunotherapy.