The mechanism of pain in the fibrositis syndrome is unknown. We measured plasma levels of substance P in 32 patients with fibrositis and 26 sex and age matched controls using a radioimmunoassay. The mean plasma level of substance P in the patients with fibrositis was 371 +/- 91 pg/ml and in controls 397 +/- 84 pg/ml (p = NS). We conclude that determination of plasma levels of substance P in fibrositis is of no diagnostic value. This does not exclude the possible role of substance P as a neurotransmitter in the fibrositis syndrome.