Performance in and knowledge of cardiopulmonary resuscitation (CPR) were assessed in a group of preclinical medical students who had received CPR certification either two or three weeks (group 0), one year (group 1), or two years (group 2) prior to the study. Assessment, ventilation, compression, and complications caused by incorrect technique were evaluated. A written examination was also given. There was significantly higher rate of failure to perform adequate CPR by students in groups 1 and 2 when compared with group 0 (p less than .05). There was no significant difference between the failure rates of groups 1 and 2. The most frequent errors related to chest compression rate and an inability to adhere to the recommended single-rescuer compression-to-ventilation ratio. Written test scores were also higher in group 0 than either group 1 or 2 (p less than .001). Written examination scores were not reliable predictors of CPR skill in individual cases.