To develop reference ranges for creatine kinase (CK) appropriate for the patient population served by this hospital, levels of serum CK were measured in 1,537 individuals in our employee population. There was substantial heterogeneity in mean, median, and range of CK levels among the several race/gender subgroups in the population studied. The race/gender subgroups could be placed into three broad groups: a high CK group, composed solely of black men; an intermediate CK group, consisting of nonblack men plus black women; and a low CK group, comprised of nonblack women. Mean CK level of the high CK group was twice that of the intermediate CK group, which, in turn, was twice that of the low CK group. Differences in mean CK values among the subgroups placed into either the intermediate CK group or the low CK group were not significant when tested with analysis of variance. Therefore, practical reference ranges for these groups are as follows: 52-520 U/L for the high CK group; 35-345 U/L for the intermediate CK group; and 25-145 U/L for the low CK group.