A prospective study of tropical pyomyositis (TP) in the Solomon Islands' Western Province followed 48 cases (mean age 10.6 years) from a population of 20,000 Melanesians over a two-year period. 32 patients were under 10 years and the male:female ratio was 1.7. Affected muscle was painful, swollen and often fluctuant but hard and indurated in presuppurative lesions. Abscesses, single in 40 and multiple in 8 subjects, were sited in the large muscles of the buttock, thigh, shoulder, arm and back on 52 occasions (90%). They contained 5-200 ml of pus which invariably grew penicillin-resistant Staphylococcus aureus sensitive to cloxacillin and erythromycin. All phage typable strains were identified as group 2 in contrast to the wider range of types found at carrier sites in otherwise healthy controls. TP was preceded by trauma in 30 cases (63%) and 26 (55%) of the patients had pre-existing pyoderma. Histological examination of clinically unaffected muscle biopsies from 10 subjects with solitary (7) or multiple (3) abscesses showed no abnormality. Serological evidence of previous infection with adenoviruses or myxoviruses was present in the same proportion (41-42%) of controls as of 22 patients tested. The results provide no evidence for antecedent diffuse myositis, viral or parasitic infections or nutritional deficiencies but support the role of trauma in localizing haematogenous skin staphylococci into damaged muscle.