In order to obtain control values for the diagnosis of carriers of Duchenne muscular dystrophy, creatine kinase (CK) estimations were performed in two laboratories on 148 healthy teenage girls (of whom 38 were premenarchal and 110 postmenarchal), 133 healthy mature women, 124 pregnant women, and 37 postmenopausal women. These levels were highest in the premenarchal teenagers, and became successively lower in the postmenarchal teenagers, the mature women, and the pregnant women, so that the mean level of the pregnant women was less than half that of the teenagers. The CK levels then rose again after the menopause. If the distribution of CK levels in adult non-pregnant women had been taken as controls for teenagers who were possible carriers for Duchenne muscular dystrophy, then one third of those teenagers classified as carriers would have been so classified incorrectly.