Different immune effector mechanisms have been characterised in the idiopathic inflammatory myopathies (IIM): in polymyositis (PM) and sporadic inclusion body myositis (sIBM), T-cell-mediated cytotoxicity targets nonnecrotic muscle fibres, whereas in dermatomyositis (DM) the complement-mediated immune response is directed against the microvasculature. As nitric oxide (NO) has an important function in cell signalling and in the cytotoxicity displayed by activated macrophages, it is potentially involved in the immunopathogenesis of IIM. Using immunohistochemical, in situ hybridisation and Western blotting techniques, we visualised the three isoforms of NO synthase (NOS) in muscle tissues from normal controls and from patients diagnosed with IIM. The levels of both constitutive isoforms of NOS (endothelial, i.e., eNOS, and neuronal, i.e., nNOS) were unchanged in IIM as compared with normal muscle. Both protein and mRNA of the inducible form (iNOS) were detected in half of the control biopsies. Constant and increased iNOS protein expression was found in endomysial infiltrates of PM and sIBM, whereas perimysial inflammatory cells in DM were largely negative. We developed a quantitative Western blotting protocol which confirmed the constitutive nature of nNOS and eNOS and the significant induction of iNOS in PM. Our results appoint iNOS with a dual function: a limited and transient role in normal muscle physiology and an active cytotoxic role in PM and sIBM.