We studied the effects of a supportive lay woman ("doula") on the length of labor and on mother-infant interaction after delivery in healthy Guatemalan primigravid women. Initial assignment of mothers to the experimental (doula) or control group was random, but controls showed a higher rate (P less than 0.001) of subsequent perinatal problems (e.g. cesarean section and meconium staining). It was necessary to admit 103 mothers to the control group and 33 to the experimental group to obtain 20 in each group with uncomplicated deliveries. In the final sample, the length of time from admission to delivery was shorter in the experimental group (8.8 vs. 19.3 hours, P less than 0.001). Mothers who had a doula present during labor were awake more after delivery (P less than 0.02) and stroked (P less than 0.001), smiled at (P less than 0.009), and talked to (P less than 0.002) their babies more than the control mothers. These observations suggest that there may be major perinatal benefits of constant human support during labor.