Utrophin is a developmentally regulated protein with considerable homology to dystrophin. It is highly expressed on fetal and regenerating fibres but in mature muscle it is confined to blood vessels and neuromuscular and myotendinous junctions. It has previously been shown that utrophin is highly expressed on mature muscle fibres in Duchenne and Becker muscular dystrophy (DMD and BMD) but the reasons for this are unclear. We have examined the immunocytochemical expression of utrophin in 42 DMD and BMD patients aged 3 months to 24 years. Patients under the age of 2 years had very little utrophin on mature fibres, but with increasing age there was considerable expression. There was no correlation between the amount of utrophin and clinical severity in either the DMD or BMD patients. All patients showed some expression of utrophin on mature fibres, although this was variable, suggesting that utrophin may be a useful diagnostic marker for distinguishing cases of BMD in which abnormalities in dystrophin are difficult to detect. Our results indicate that the abnormal expression of utrophin in DMD and BMD is not a continuation of expression following fetal development or regeneration, but is an age-related secondary event. Expression of utrophin in these patients does not correlate with severity and the levels observed in these patients do not appear to be of clinical benefit. Thus, any therapeutic advantage of utrophin in human dystrophic muscle still has to be evaluated.