We evaluated whether elderly patients with pulmonary tuberculosis and without apparent preexisting liver disease are at an increased risk to develop hepatotoxicity from an isoniazid-rifampin regimen and require regular liver function tests in comparison with younger patients. We analyzed the data of 131 patients treated in the period 1980-1985 of whom 64 (49%) were at least 60 yr of age. Subsequent increases of transaminases (measured weekly for as long as 4 wk after the start of treatment and later on when symptoms suggestive of hepatotoxicity occurred) above baseline values were found more frequently in the elderly (38 versus 18%, p < 0.05) and were also more pronounced in them (p < 0.01). The ratio of the highest transaminase value over the baseline value was called the transaminase index (TI). A TI of at least 5 was found in 22% of elderly and 8% of younger patients (p = NS), but ratios as high as 10 were mostly asymptomatic and always normalized progressively without treatment adjustment. Only symptomatic patients with a TI > or = 10 (five elderly and three younger) required temporary or definitive treatment adjustment. We conclude that repeated liver function test evaluations are generally unnecessary, except for symptomatic elderly and younger patients alike, in order to detect those with a TI > or = 10, thus requiring drug adjustment.