Skeletal muscle damage size (SMDS) was assessed in 35 women and 34 men runners after a 42.2-km race using a method developed for estimation of myocardial infarct size. SMDS was computed according to the following equation: SMDS = (BW) (K) (CKr), where BW is body weight, K is a constant, and CKr is the cumulative amount of creatine kinase (CK) released over time. The method takes into account CK distribution space, fractional disappearance rate of CK, proportion of CK degraded in skeletal muscle, and proportion of CK released into the circulation. Assumptions are made regarding the relative amount of CK lost from skeletal muscle into the circulation. The SMDS in men, 808 +/- 1,229 (SD) CK g-eq was significantly (P less than 0.05) greater than in women, 160 +/- 147 (SD) CK g-eq. The ranges of SMDS (CK g-eq) were 23-5,397 in men and 7-624 in women. A significant difference (P less than 0.05) also remained after correction for body surface area; men 432 +/- 583 (SD), women 100 +/- 63 (SD) CK g-eq/m2. In men and women, no significant correlation existed between SMDS and age or marathon finish time. Although relatively theoretical, results indicate that greater skeletal muscle damage occurred in men vs. women runners after a marathon. Whether the release of CK from skeletal muscle is the result of irreversible and/or reversible injury has not yet been determined.