Two hundred forty-four Toronto pedigrees of Duchenne muscular dystrophy patients have been partitioned into nuclear families with pointers for complex segregation analysis under a mixed model. The model takes into account the major X-linked locus and a multifactorial transmissible component for creatine kinase activity in females. The incidence in the province of Ontario is estimated to be 292 per million male births. The proportion of sporadic cases is 1/3, demonstrating equal mutation rates in males and females. A multifactorial component (H = 0.379) contributes to family resemblance for creatine kinase measurements. Examples are presented of the application of a computer program, COUNSEL, to derive genetic risks for genetic counseling with consideration of the multifactorial component.