After adjustment for age and other factors, the relative risk of malignant lymphoma for cats with any household ETS exposure was 2.4 (95 percent confidence interval: 1.2, 4.5). Risk increased with both duration and quantity of exposure, with evidence of a linear trend. Cats with 5 or more years of ETS exposure had a relative risk of 3.2 (95 percent confidence interval: 1.5, 6.9; p for trend = 0.003) compared with those in nonsmoking households. These findings suggest that passive smoking may increase the risk of malignant lymphoma in cats and that further study of this relation in humans is warranted. Am J Epidemiol 2002;156:268–73.