We compared the maternal and birth characteristics of 87 first-year dropouts with 103 active participants of a pediatric clinic which provides long-term follow-up care to offspring exposedin uteroto substance abuse mothers. The age, ethnicity, and marital status of the mother; the type of drug used during pregnancy; and the length, gender, number, and type of addiction problems of the newborn were not significantly different between the two groups. However, the dropouts differed significantly from the active participants in many other aspects. More dropout mothers received no prenatal care and did not have children at home at the time of the present birth. More of them were smokers; had gonorrhea, heart, and kidney problems; and had infections complicating their pregnancy. The dropout newborns were found to have lower birthweight and shorter gestation. These clinical parameters defined a suboptimal group of mothers and children requiring more attention and care. The findings also alert investigators of long-term follow-up studies to be aware of basic differences between nonparticipants and participants.