Summary Simultaneous measurements of muscle energy metabolism using 31P-magnetic resonance spectroscopy (31P-MRS) and the kinetics of muscular oxygen metabolism using near-infrared spectroscopy (NIRS) were conducted in polymyositis (PM) patients. The subjects were 12 PM patients (age 45 ± 12 years) and 12 normal controls (age 41 ± 12 years). The muscle phosphocreatine (PCr) index and intracellular pH (pHi) were determined with 31P-MRS and the changes in intramuscular oxygenated (oxy-Hb), deoxygenated (deoxy-Hb), and total haemoglobin (total Hb) were evaluated with NIRS . The pHi and PCr index before steroid therapy in PM patients were significantly lower during exercise than in normal controls, and their recovery was statistically significantly delayed compared with the controls. The pattern of changes in NIRS over time before steroid therapy in PM patients differed from that in normal controls. There were smaller changes in deoxy-Hb and oxy-Hb during exercise, and total Hb decreased during exercise. In contrast, the kinetics of muscular metabolism after steroid therapy showed changes similar to those seen in normal controls. Simultaneous 31P-MRS and NIRS measurements to determine the kinetics of muscular metabolism are expected to be useful as a noninvasive approach for the evaluation of treatment effects in PM patients.