The striking proliferation of connective tissue in Duchenne muscular dystrophy is attributed, besides other components of the extracellular matrix, to an increase of endomysial and perimysial type III and type I collagen. We investigated if muscle fibrosis correlates to an increased serum concentration of procollagen I or III. Therefore, we measured the serum levels of carboxyterminal propeptide of type I procollagen, aminoterminal propeptide of type III procollagen and laminin P1 in 20 boys with progressive muscular dystrophy (16 definite Duchenne muscular dystrophy, 2 suspected of Duchenne muscular dystrophy, 2 Becker muscular dystrophy). In contrast to collagen I and III the expression of laminin in the basement membrane is known to be normal in Duchenne muscular dystrophy. There was no significant alteration of serum concentration of procollagen III N-peptide, procollagen I C-peptide and laminin P1 in boys with Duchenne muscular dystrophy. Measuring these parameters is not useful for investigating the extent of muscle fibrosis or for monitoring the effect of therapeutic trials such as steroid treatment.