Some validity aspects related to various sources on pregnancy outcome and exposure are discussed. Register-based data on outcome and exposure are compared with workers' own reports. The problems in the use of personal interview data on pregnancies are related to the possible selection in recognition and reporting the pregnancy outcome. The best way of avoiding misclassification of outcome is to resort to medical records whenever possible. Misclassification of exposure is most likely a common reason for discrepancies in results in reproductive studies. The misclassification of exposure deflates the power of the study to detect the true difference between exposed and nonexposed subjects, particularly when the prevalence of exposure is low. It is of great importance to confirm, if possible, the exposure status by using two or more independent sources of data.