If interviewers' personal habits or attitudes influence respondents' answers to given questions, this may lead to bias, which should be taken into consideration when analyzing data. The authors examined a potential interviewer effect in a study of pregnant women in which exposure data were obtained through computer-assisted telephone interviews. The authors compared interviewer characteristics for 34 interviewers with the responses they obtained in 12,910 interviews carried out for the Danish National Birth Cohort Study. Response data on smoking and alcohol consumption in the first trimester of pregnancy were collected during the time period October 1, 1997-February 1, 1999. Overall, the authors found little evidence to suggest that interviewers' personal habits or attitudes toward smoking and alcohol consumption during pregnancy had consequences for the responses they obtained; neither did the interviewers' education, age, or parity correlate with the answers they obtained. In these data gathered through computer-assisted telephone interviews, interviewer effects arising from variations in interviewers' health beliefs and personal habits were found to be negligible. Thorough training of the interviewers and continuous supervision may have contributed to this finding.