The acute phase response (APR) was studied longitudinally in 12 episodes of gout and two episodes of pscudogout. The serum C-rcactivc protein (CRP) was elevated in all episodes (peak concentrations 24.8–542 2 ⊸g/ml), fell with treatment, and correlated with number of joints involved CRP was detectable in all synovial fluids. Prealbumin showed a negative APR Serum interleukin-1 (IL-I) activity was detected in sera during an episode of acute crystal synovitis in all patients studied, and was present in all synovial fluids assayed. This marked acute phase response may suggest macrophagc activation as an important mechanism in acute crystal synovitis.