Epidemiological and bacteriological aspects of human Mycobacterium bovis disease were investigated in south-west Ireland (counties Cork & Kerry, population 536,000) over the years 1983-92 inclusive and compared to M. tuberculosis. Results showed a small, stable incidence of culture positive M. bovis human disease, mean annual incidence 0.56 per 100,000 population compared to a higher but declining incidence of culture positive M. tuberculosis (15.3 per 100,000 in 1983, 9.0 per 100,000 in 1992). Male patients were the majority, 63.4 per cent of M. bovis; 62.4% of M. tuberculosis (p = 0.03). Fifty three per cent of M. bovis cases (n = 30) were pulmonary, compared to 85% of M. tuberculosis (n = 626; p = 0.0001). M. bovis patients were older (p = 0.02), mean age 58.4 years (SD 18.9) compared to 48.5 (SD 22.2). The mycobacterial smear positive rate was similar in both groups taken as a whole. No rural-urban difference in incidence was found in either disease, suggesting in the case of M. bovis initial infection in childhood via contaminated milk in the pre-pasteurisation era.