OBJECTIVE: To determine blood serum creatine kinase (CK) levels in football players undergoing 2-a-day football practices and to determine if CK levels are related to fitness. Our hypotheses were that CK levels in each subject would increase over the course of practices and that higher levels of fitness would result in smaller increases in CK. DESIGN AND SETTING: Creatine kinase measurements were taken 4 times over 10 days of preseason, 2-a-day practices: before beginning practices (CKM1) and on the mornings of the 4th (CKM2), 7th (CKM3), and 10th (CKM4) days of practice. SUBJECTS: Twelve male Division I football players from a midwestern university. MEASUREMENTS: Fitness tests included percentage of body fat, body mass index, anaerobic capacity, and peak power from a 1-leg step test and 1-repetition maximum bench press and squat lifts. Changes in CK levels were calculated as the difference between the second CK measure (CKM2) and the first CK measure (CKM1). RESULTS: Differences were significant between the CK measurements (P =.0002). Post hoc analysis revealed that CKM2 and CKM3 levels were statistically higher than CKM1 levels. No other statistically significant differences between CK measures were noted. Pearson product moment correlation coefficients showed that athletes who generated higher peak power during a 15-second step test had smaller increases in CK levels from CMK1 to CMK2 (r = -.64). Although the correlations with anaerobic capacity (r = -.54, P =.071), body mass index (r = -.51, P =.090), and percentage of body fat (r = -.52, P =.082) approached statistical significance, no other correlations were statistically significant. The mean CKM2 level was 5124.7 U.L(-1) +/- 5518.1, approximately 30 times the norm for men. CONCLUSIONS: Participation in 2-a-day football practices resulted in significant serum CK elevations, which remained elevated for at least 7 days. Participants who had higher peak power had smaller increases in CK.