A random sample of 276 people representing control, direct exposure, and probable indirect exposure in the Goiânia, Brazil radiological accident was examined using micronuclei as indicators of cytogenetic damage. The Goiânia subjects were analyzed for interactions of age, lifestyle, and ionizing radiation dose. Increases in micronucleus frequencies were most strongly correlated with the dose of ionizing radiation, but age, alcohol consumption, and smoking habits also affected micronucleus frequencies. Despite these additional influences, micronucleus frequencies can be useful as biological dosimeters.