The plasma myoglobin concentration was measured before and after massage of 26 patients with myofascial pain. Twenty-one patients had a successful treatment and a significant increase was observed in the plasma myoglobin concentration (median 125 micrograms/l, range 35-439) within a maximum of 2 hours after the first massage treatment (p less than 0.0001). A positive correlation was found between the degree of muscle tension and pain, and the increase in plasma myoglobin concentration. After repeated massage treatment a gradual decline in the increase in plasma myoglobin concentration could be demonstrated parallel to a reduction in the muscle tension and pain. Five patients did not benefit from massage treatment and no significant increase in the myoglobin in plasma was measured. These patients were in pain and had a high degree of muscle tension. The observed increase in myoglobin in plasma after massage indicates a leak of myoglobin from the muscle fibres in 21 patients, whose myofascial pain seem to be linked with a muscle fibre disease. It is suggested that 5 patients with the same muscle symptoms have another, still unknown muscle disease.