OBJECTIVES: To examine the association between HIV infection and tuberculosis (TB) and the proportion of TB attributable to HIV in Chiang Rai province, northern Thailand, in response to a rapidly changing incidence of HIV infection. METHODS: Case-control study covering the period 1990 to 1998. Cases were all new TB patients seen at Chiang Rai Hospital. Controls were antenatal clinic attendees, delivery patients, surgical patients, blood donors, and military conscripts. Odds ratios (ORs) were calculated by year, age group, and sex, using each control group separately. The population attributable fraction was calculated by year. RESULTS: During the study period, the number of new TB cases in Chiang Rai Hospital increased more than threefold. The ORs increased over time compared with all control groups for both sexes but did not vary consistently with age. The proportion of TB cases attributable to HIV rose to 72.0% in male patients and 65.8% in female patients by 1998. CONCLUSIONS: The HIV epidemic has a profound and prolonged impact on TB burden. Despite the marked reduction in HIV incidence already seen in Chiang Rai, the HIV prevalence among TB cases and the proportion of cases attributable to HIV continue to rise.