The effect of maternal and environmental factors on the developmental outcome of infants with low medical risk born to adolescent and non-adolescent, primiparous, Caucasian mothers was studied. Twenty-one were adolescent mothers (less than or equal to 17 years of age) and 21 were non-adolescent (21-29 years of age). Assessment of child care support, life stress, and home environment were carried out at 4 months. Infants were examined at 8 months, and child care support and life stress were reassessed. Adolescent mothers reported less child care support at both 4 and 8 months and more life stress at 8 months than non-adolescent mothers. Adolescent mothers were also found to be less responsive, used restriction and punishment more often, and were less involved with their infants during the home observation. Infants of adolescent mothers had lower Bayley Mental Development Indices at 8 months. A theoretical model, whereby the mother's age, education, and socioeconomic status (SES) were conducive to less optimal child care support and life stress, which in turn affected the infant's cognitive status, was supported by this data. Thus, within a low medical risk population, we have documented the significant role of maternal and environmental factors in determining the infant's cognitive status.