The objective of this study was to identify characteristics of nonrespondents and late respondents in a population-based case-control study on lung cancer conducted in the city of Turin (Italy). Information about demographic and socioeconomic variables of 634 cases and 859 controls who responded to an interview, as well as of 154 cases and 154 controls who did not respond were obtained from census and the public register of Turin. The socioeconomic level of nonrespondents was high in cases but low in controls. Late respondent controls (i.e., individuals who responded after contact through their general practitioner) had socioeconomic characteristics comparable with those of nonrespondents, while they were similar to respondents with respect to demographic variables. The interview of late respondents halved, from 14 to 7%, the magnitude of the bias introduced by nonresponse on the estimate of the association between educational level and lung cancer. Nonresponse, associated with socioeconomic status, is an important potential source of bias in population-based case-control studies, which should always be considered and discussed. The direction and magnitude of the bias can be quantified. General practitioners may contribute to decrease nonresponse bias. Caution should be used in inferring characteristics of nonrespondents on the basis of those of late respondents.