OBJECTIVE To review the clinicopathologic features and outcome of sporadic late onset nemaline myopathy (SLONM). BACKGROUND Non-HIV-related SLONM is an uncommon disease of undefined etiology. METHODS This study is based on clinical, EMG, histochemical, immunocytochemical, and electron microscopy evaluation, and long-term follow-up of 14 patients observed at the Mayo Clinic between 1975 and 2003. RESULTS The disease presented between 43 and 81 years and evolved subacutely. The weakness was predominantly proximal in 11, equal proximally and distally in 3, and asymmetric in 4; dysphagia was a symptom in 6. The EMG showed myopathic features with fibrillations but the serum CK level at the time of initial examination or reevaluation was normal or below the Mayo Clinic's range of normal values for sex and age at the time of the assay. Seven patients had an associated monoclonal gammopathy. On light microscopy, the nemaline structures were best identified in 3-mum-thick frozen sections stained trichromatically or immunostained for alpha-actinin or myotilin. Electron microscopy done in 12 cases identified the rods in all and revealed additional structural abnormalities. Seven patients with monoclonal gammopathy were followed for 1 to 5 years; five died of respiratory failure. Five patients without monoclonal gammopathy were followed for 4 to 23 years and none died of the disease. Immunotherapy in eight patients was of uncertain benefit. CONCLUSIONS 1) Subacutely evolving weakness after age 40, normal to low CK level, myopathic EMG with fibrillations, and often a monoclonal gammopathy are clues for the diagnosis of sporadic late onset nemaline myopathy. 2) The diagnosis is confirmed by visualizing the rods in trichrome or immunostained cryosections. 3) An associated monoclonal gammopathy heralds an unfavorable prognosis.