Neuromuscular manifestations, including a variety of peripheral neuropathies and myopathies, can occur either as isolated clinical phenomena or concurrently with other clinical manifestations, and are common in acquired immunodeficiency syndrome (AIDS). A 33-year-old male patient had a four-year history of increasing proximal muscle weakness of the extremities. Elevated muscle enzymes, a muscle biopsy and interstitial lung disease all suggested a diagnosis of polymyositis with lung involvement. However, a transbronchial lung biopsy revealed Pneumocystis carinii pneumonia, and serology for HIV infection disclosed positive results. Zidovudine and cotrimoxazole were instituted and the patient improved. HIV-associated myopathy should be considered by practicing clinicians in cases presenting with myopathy, since the number of patients with HIV infection is increasing in Taiwan.