Recently, clinical follow-up studies have shown that the age of first symptoms and the speed of progress may vary considerably in Duchenne muscular dystrophy (DMD). Prognostic factors would be of interest not only for the patients themselves, but also for the selection of DMD patients for therapeutic studies. Creatine kinase (CK) activity reflects the dystrophic degeneration of the muscle cells. Therefore, the coherence of the course of CK activity and its relationship to the clinical state was investigated with the aid of a heuristic mathematical formula. Two constants were calculated, b and c. The data of 88 DMD patients proved the validity of the mathematical formula. Constant b seemed to be of no clinical relevance. The height of constant c was related to the age at which the ability to walk was lost (r = -0.76). This constant provides an objective individual assessment of the prognosis in DMD. In addition, the mathematical analysis of the CK course opens the way for a prospective CK estimation, which is relevant for the evaluation of therapeutic trials.