The prevalence of Duchenne muscular dystrophy in the North of England (population 3.07 million) was 4.0 x 10(-5) in 1968, 3.6 x 10(-5) in 1979 and at least 2.3 x 10(-5) in 1988. The incidence in males was 31.2, 22.4 and at least 17.8 x 10(-5) in successive decades since 1952. Thirty-five percent of cases over the last 35 years have been preventable; but recently major additional educational programmes and routine screening tests for presymptomatic cases would have been required to improve prevention. The mortality has not changed in the last 20 years. Previously published criteria for the diagnosis of autosomal recessive muscular dystrophy in childhood have been tested prospectively. Seven new children fitting these criteria and 6 other girls with muscular dystrophy are described. Only 5 appear to have the autosomal recessive disease. Two boys had DNA deletions and 2 girls were severely manifesting carriers of the Duchenne gene.