The private sector's role in increasing the use of maternal health care for the poor in developing countries has received increasing attention, yet few data exist for urban slums. Using household-survey data from 1,926 mothers in two informal settlements in Nairobi, Kenya, collected in 2006, we describe and examine the factors associated with women's use of private and government health facilities for childbirth. More women gave birth at private facilities located in the settlements than at government facilities, and one-third of the women gave birth at home or with the assistance of a traditional birth attendant. In multivariate models, women's education, ethnic group, and household wealth were associated with institutional deliveries, especially in government hospitals. Residents in the more disadvantaged settlement were more likely than those in the better-off settlement to give birth in private facilities. In urban areas, maternal health services in both the government and private sectors should be strengthened, and efforts made to reach out to women who give birth at home.