We investigated the possible influence of aryl hydrocarbon hydroxylase (AHH) on susceptibility to bladder cancer in humans. AHH inducibility was measured in the cultured lymphocytes of 16 patients who were being followed after successful treatment for bladder cancer, in 53 progeny of bladder cancer patients, and in matched controls. In both the progeny and patient populations, no evidence was found for a difference between the distribution of AHH inducibility or induced AHH activity compared to the distribution among control individuals. Thus, AHH acitivity or inducibility does not appear to be a major determinant of bladder cancer risk in humans.