We compared the transmission rate of Chlamydia trachomatis infection from infected women to their infants after various modes of delivery. After vaginal birth, Chlamydia trachomatis was isolated from 58 of 125 infants with a cephalic presentation, and serological evidence of chlamydial infection was found in another eight. C. trachomatis was isolated from the only infant with a frank breech presentation. After Caesarean birth, C. trachomatis was isolated from two of 10 infants born after rupture of the membranes and from one of six without prior rupture of the membranes. No serological evidence of infection was found in any of the culture-negative infants born by Caesarean section. By survival analysis, rates of transmission were significantly lower after Caesarean section with rupture of the membranes before delivery than after vaginal delivery. Infants born to infected women are at risk of C. trachomatis infection regardless of route of delivery.