We estimate the impact of family structure on investments made in children's health, using data from the 1988 National Health Interview Survey Child Health Supplement. Controlling for household size, income and characteristics, we find that children living with step mothers are significantly less likely to have routine doctor and dentist visits, or to have a place for usual medical care, or for sick care. If children living with step mothers have regular contact with their birth mothers, however, their health care does not suffer relative to that reported for children who reside with their birth mothers. In addition to health investments, we find a significant effect of step mothers on health-related behaviors: children living with step mothers are significantly less likely to wear seatbelts, and are significantly more likely to be living with a cigarette smoker. We cannot reject that investments for children living with birth fathers and step mothers are the same as those made by birth fathers living alone with their children. Who invests in children's health? It appears these investments are made, largely, by a child's mother, and that step mothers are not substitutes for birth mothers in this domain.