Pregnancy-induced hypertension affects at least 10% of all pregnancies. An association with first pregnancy or a change in paternity for subsequent pregnancies has been suggested. We studied the duration of sexual cohabitation with the father prior to conception and the incidence of pregnancy-induced hypertension. During a five-month period, 1011 consecutive women who delivered in an obstetric unit were interviewed about paternity and duration of sexual cohabitation before conception. Obstetric charts were abstracted to identify three groups: those with pregnancy-induced hypertension, chronic hypertension, and normal blood pressure. The incidence of pregnancy-induced hypertension was 11.9% among primigravidae, 4.7% among same-paternity multigravidae, and 24.0% among new-paternity multigravidae. For both primigravidae and multigravidae, length of sexual cohabitation before conception was inversely related to the incidence of pregnancy-induced hypertension (p < 0.0001). Similar results were observed after control for race, education, maternal age, marital status, and number of pregnancies. Pregnancy-induced hypertension may be a problem of primipaternity rather than primigravidity. Furthermore, an extended duration of sexual cohabitation before conception may protect against pregnancy-induced hypertension.