Biopsy specimens were taken from the affected first dorsal interosseous muscle in 29 women with painful chronic overuse syndrome and from 8 volunteer controls. Structural differences in samples from the women with overuse syndrome included increased type 1 fibres with type grouping, decreased type 2 fibres, type 2 fibre hypertrophy, increased internal nuclear count, mitochondrial changes, and various ultrastructural abnormalities. These changes were related to clinical severity, and point to an organic cause for the syndrome.