Persistence of monosodium urate crystals and low-grade inflammation in the synovial fluid of patients with untreated gout. Synovial fluid (SF) from 74 asymptomatic knees of patients with gout was analyzed. These patients had never been treated with serum uric acid-lowering agents. SF samples were analyzed for the presence of crystals and for cell counts, using undiluted SF directly in a Niebauer cell counting chamber. Thirty-seven of the aspirated knees had previously been inflamed, and monosodium urate (MSU) crystals were seen in SF aspirates from 36 of them (97%). Of the 37 knees that had never been inflamed, only 8 contained MSU crystals (P less than 0.00001). After reaching the joint, MSU crystals will persist in the SF as long as the serum uric acid level has not been lowered by appropriate treatment. In this situation, analysis of SF is an excellent diagnostic test for the intercritical gout. The mean cell count in the 44 SF samples that contained MSU crystals was 449/mm3 (95% confidence interval 310-589); the mean cell count in the 30 SF without MSU crystals was 64/mm3 (95% confidence interval 34-95) (P less than 0.00002). The SF samples that contained MSU crystals also had a higher percentage of polymorphonuclear leukocytes than those without MSU crystals (P less than 0.002). These data suggest that there is a low-grade inflammation present in the knees of patients with MSU crystals, the consequences of which should be investigated. 