Little attention has been given to the relationship between fathers' use of psychoactive substances and the biologic and social development of the newborn. In a longitudinal study of prenatal cocaine exposure, characteristics of the father of the index child were ascertained by post-partum interview from 252 mothers at Boston Medical Center. Fathers were primarily African American (65%) or African Caribbean (21%), mean age was 29 years (s.d. 7), with no education beyond high school (87%). 22% were reported to have a drug and/or alcohol problem. Heavy (top quartile) cocaine using mothers were 1.6 times more likely than lighter users and 2.6 times more likely than non-users to report that the father had a history of a drug and/or alcohol problem(p=.006). Fathers' perceived drug and/or alcohol problem was associated with maternal report of physical or sexual abuse (p=.001) and emotional abuse(p=.025) during the index pregnancy, and with the mothers' identification of the father as the perpetrator of the abuse (p=.001). Mothers' perception of a paternal drug and/or alcohol problem was also associated with fathers' lifetime history of incarceration (p=.001) and unemployment at the time of the delivery (p=.04). There was a trend (p=.15) for fathers' perceived drug and/or alcohol problem to be associated with lower infant birthweight and shorter length after controlling for maternal characteristics. Paradoxically, fathers' perceived drug and/or alcohol problem was associated with fathers providing care more frequently to their infants at six months of age (p=.04). Mothers' heavy cocaine use is strongly associated with an increased likelihood of mothers' reporting a paternal drug and/or alcohol problem. Therefore, childhood cognitive and behavioral outcomes currently attributed to maternal substance use in pregnancy may also reflect the genetic, teratogenic, and social influence of fathers, particularly of fathers' substance abuse.