Blood concentrations of hydrogen sulfide (H2S) are markedly elevated in several animal models of inflammation. Pharmacological inhibition of H2S synthesis reduces inflammation and swelling, suggesting that H2S is a potential inflammatory mediator. However, it is currently unknown whether H2S synthesis is perturbed in human inflammatory conditions or whether H2S is present in synovial fluid. We analyzed paired plasma and synovial fluid (SF) aspirates from rheumatoid arthritis (RA; n= 20) and osteoarthritis (OA; n= 4) patients and plasma from age matched healthy volunteers (n= 20). Median plasma H2S concentrations from healthy volunteers and RA and OA patients were 37.6, 36.6, and 37.6 μM, respectively. In RA patients, median synovial fluid H2S levels (62.4 μM) were significantly higher than paired plasma (P= 0.002) and significantly higher than in synovial fluid from OA patients (25.1 μM; P= 0.009). SF H2S levels correlated with clinical indices of disease activity (tender joint count, r= 0.651; P < 0.05) and markers of chronic inflammation; Europhile count (r=−0.566; P < 0.01) and total white cell count (r=−0.703; P < 0.01). Our study shows for the first time that H2S is present in synovial fluid and levels correlated with inflammatory and clinical indices in RA patients.