Case-control studies of prenatal risk factors for disease in later life often ascertain cases from within a defined area, trace the birth records of those cases born within the area, and select controls from birth records within the same area. Bias can occur in these studies if the disease risk factors are related to migration from the area. The effects of this bias were examined in a study in Oxfordshire, England. Cases (n = 218) of diabetes in children and young adults born during 1965-1986 were identified from hospital discharges during 1965-1986; controls (n = 753) were selected from livebirths during 1965-1986. By 1987, 219 controls (29.1%) had migrated from Oxfordshire or died. Low maternal parity and high social class were strongly related to migration, more than the other perinatal factors studied. Migration, therefore, could lead to apparent associations of diabetes risk with parity or social class. For a general instance, the authors show how much bias is caused by different degrees of migration and of association between migration and a perinatal risk factor. Examples are given of how migration can produce apparent trends in risk as well as increased or decreased individual relative risks. If more than 25% of controls migrate, bias may be appreciable.