Of a total study group of 99 patients 56 received intrapleural BCG in three different dosages (5 patients 16 X 10(6), 25 patients 32 X 10(6), and 26 patients 64 X 10(6) culturable particles) following resection treatment for lung cancer. This study group was compared with an historical control population, very closely matched with regard to age, sex, stage of disease, histology and type of operation. Although the clinical condition of the patients selected for BCG treatment was above average, no survival benefit ensued from the intrapleural BCG. On the contrary, disease-free survival in BCG receivers with stage I and II squamous cell carcinoma was shortened significantly at 2 years follow-up due to the earlier appearance of local recurrences, with the same tendency still present after 5 years. This observation suggests an enhancement of tumour growth by the intrapleural BCG treatment. This alarming phenomenon could be a warning to clinicians when planning clinical immunotherapy trials to be aware of the potentially deleterious effects of such treatment.