Bilateral open biopsies from the painful upper part of the trapezius muscle were studied in 17 patients with localized chronic myalgia related to static load during repetitive assembly work. Isolated pathologic ragged red fibers were related to the presence of myalgia. The phenomenon indicating disturbed mitochondrial function was confined to the Type 1 fibers. Using a laser-Doppler flowmeter, the muscle blood flow was recorded in the exposed muscle before a biopsy was taken. Pain was assessed and graded as the difference between the two sides, as was the presence of ragged red fibers. The myalgia correlated with reduced local blood flow: the greater the pain difference, the greater the reduction in blood flow. There was a correlation between the presence of mitochondrial changes and reduced muscle blood flow.