The frequency of the fracture of clavicle in liveborn infants delivered in the Obstetric Department of the School of Medicine of Sassari was retrospectively evaluated for the years 1977, 1978 and 1979. Moreover, all the fractured live infants born in 1980 were prospectively identified and compared to a group of unmatched controls, which included the live infant born immediately before and that born immediately after the index case. The same procedure was adopted when the fracture was identified in two infants born consecutively. The following variables were studied: date and hour of delivery, sex, birthweight, gestational age, type of labour, presentation and mode of delivery, Apgar at one minute, parity of the mother. There were 108 cases and 194 controls. The frequency of the fracture of clavicle showed a linear increase from 2.2/100 live births in 1977 to 4.8/100 live births in 1980; males were slightly more affected than females and the right clavicle was more frequently fractured than the left one (67% for the four years altogether). In the prospective study the only statistically significant differences between cases and controls were found for birthweight (less than 0.001) and mode of delivery (P less than 0.025): cases weighed more and were more frequently born by instrumental delivery (vacuum or forceps) than controls; none of the fractured infants was born by caesarean section. One of the fractured infants was affected by osteogenesis imperfecta and two other presented a transitory paresis of the omolateral brachial plexus; in the remaining cases the prognosis was good.