Objective—To describe several aspects of booster seat use and misuse in a sample of children attending child safety seat clinics. Methods—Booster seat practices were assessed at 76 child safety seat clinics held between April 1997 and January 1999 in Pennsylvania and southern New Jersey. At each assessment, a child passenger safety team evaluated the booster seat and identified modes of misuse. Results—Altogether 227 booster seats were observed. Sixty eight per cent (68%) of shield boosters and 20% of belt positioning boosters were misused. Thirty two per cent of the children using a shield booster weighed more than 40 lb (18.1 kg); 68% of children in shield boosters and 63% in belt positioning boosters weighed less than 40 lb. Conclusions—This study identified a relatively high rate of booster seat misuse. Shield boosters were more likely to be misused than belt positioning booster seats. Significant numbers of children weighing more than 40 lb were using possibly dangerous shield boosters. The majority of children in this study were less than 40 lb. In this weight range, a convertible child restraint system provides better protection than a booster seat. Booster seat use should only be initiated once the child has completely outgrown their convertible child restraint system.