Little information is available about bicycle-mounted child seats despite their general use for a decade. We analyzed two existing data sets to gain information about frequency, trend, and characteristics of bicycle-mounted child seat injuries to children 5 years old and younger. Available data suggest an increased frequency of these injuries, with the rate of passenger injuries rising from 17% to 28% of all reported bicycle-related injuries to children in this age group in California during the years 1977 to 1986. In a detailed sample of 52 injuries related to the use of bicycle-mounted child seats, 42% occurred when the bicycle crashed or tipped over and 25% occurred when the child fell out of the seat. Sixty-five percent involved the head and face, and 27% of the head injuries were serious. Substantial morbidity associated with these injuries could be amellorated if children using these seats wore appropriate bicycle helmets.