Experimental data have demonstrated that chronic infection with intracellular parasites may enhance resistance against some types of tumour. This phenomenon has not yet been demonstrated for experimental Trypanosoma cruzi chronic infection. This study investigated the effect of a specific colon cancer inducing drug, 1,2-dimethylhydrazine (DMH), on chronically T.cruzi infected Wistar rats. Infection was obtained by inoculation of 10(5) tripomastigote forms by subcutaneous (s.c.) route. Acute phase of the infection was monitored every other day by examination of a blood smear from each animal until negativation. In the early chronic phase of the infection, colon adenocarcinoma was induced by weekly s.c. injections of DMH at a dose of 20 mg/kg body weight for 12 weeks. 102 animals were divided in four test groups: 39 infected rats received DMH (group 1); 32 non-infected rats received DMH (group 2); 16 infected rats and 15 non-infected animals were used as control groups. Animals were killed 6 months after the first dose of DMH. The whole colon was removed and prepared for light microscopic examination. Twelve animals from group 1 and 22 from group 2 had colon adenocarcinomas, the proportion of cancer being 30.7 and 68.7%, respectively (chi(2) = 10.16; P < 0.05). The relative risk of having a colon tumor in infected animals (group 1) was 0.45 (IC 95% 0.26-0.76), which is a protective risk compared with non-infected animals. These findings show that chronic infection with T.cruzi is associated with a lower incidence of DMH-induced colon cancer in rats.