In ecologic studies, covariate levels of groups are often quantified as the prevalence of a dichotomous covariate. We show that, under certain conditions, nondifferential misclassification of such a binary covariate does not reduce the ability to control confounding by the covariate in ecologic studies. Thus, any remaining exposure-disease association in an adjusted ecologic analysis cannot be ascribed to incomplete control for confounding due to nondifferential misclassification of the dichotomy under those conditions, although residual confounding by the underlying covariate may still be present. This point is illustrated by ecologic analyses of the association between population density and mortality from lung cancer in women in 30 administrative districts of the Federal Republic of Germany, in which control for cigarette smoking is required.