Fifty-four patients with evidence of locally advanced primary squamous cell bronchial carcinoma (SCC), and three patients with adenocarcinoma (AC) had lung resection to remove all the visible tumour. After operation an randomly chosen group of 20 SCC patients received adjuvant BCG immunostimulation by scarifications (BCG-A). An additional group of 14 SCC patients, and three AC patients received initially intrapleural BCG treatment and subsequently scarifications (BCG-B). A control group of 20 SCC patients received no adjuvant treatment. Follow-up studies were done from three to 51 months. Immune reactivity was monitored in vivo with PPD skin tests in 33 treated and in 18 untreated patients. In both the BCG-treated SCC groups recurrence rates decreased statistically significant during follow-up (BCG-A; six to 51 months, p less than 0.001; BCG: 6-9 months, p less than 0.01 and nine to 24 months, p less than 0.001). However, no difference could be demonstrated between systemic and combined systemic and intrapleural treatment. The three BCG-treated AC patients all relapsed within nine months of follow-up. A pronounced increase in skin reactivity to PPD was seen six months after surgery in the BCG-treated patients (BCG-A, p less than 0.001; BCG-B, p less than 0.01), whereas the control patients remained anergic after surgery. This improved immune reactivity went in parallel with a more favourable outcome of the individual patients (BCG-A, p less than 0.02; BCG-B, p less than 0.05). It is concluded that adjuvant BCG immunotherapy used in strongly selected patients with minimal residual squamous cell bronchial carcinoma improves the prognosis. Intrapleural treatment did not improve the prognosis further. A favourable clinical outcome was mirrored by an increase in cellular immune reactivity.