The mean levels of serum creatinine phosphokinase (CPK) were studied in three groups of women: normal controls (57), obligate carriers for Duchenne muscular dystrophy (30), and mothers of isolated cases of this disease (35). The distribution of the levels in these groups was significantly different and was in keeping with the hypothesis that one-third of isolated cases result from new mutations. The control and carrier ranges overlapped considerably, with the level of CPK of 33% of obligate carriers coming within the 97 1/2 centile of the normal range. Odds against an individual being a carrier were derived for specific mean values of CPK. They should be considered with genetic information using Bayes's theorem. The mean CPK levels in obligate carriers showed significant familial clustering. This may have implications in carrier detection.