In order to investigate the effect of natural infection with M. tuberculosis on cancer incidence, a case-control study was undertaken on 108 recently diagnosed cancer cases (all sites) and 317 controls. After removal of cancer cases or controls considered immunoincompetent (25% and 17% respectively), the remaining subjects were tested by intradermal injection of tuberculin (5 I.U.). Results were compared separately for each sex. It was observed that the proportion of tuberculin-negative subjects was higher in cancer patients than in controls within each sex (males 31.3% vs 20.1%, females: 59.2% vs 37.3%) the difference in females being statistically significant (p less than 0.02). Odds ratio for negative tuberculin test was 1.80 and 2.44 for males and females respectively. The results are discussed on the basis of cell-mediated immunity produced by infection with M. tuberculosis and its relations with tumor growth.