Synovial fluid white blood cell counts are considered to be useful in diagnosing infectious arthritis, however, considerable overlap exists between infectious and noninfectious types of inflammatory arthritis. We undertook this review of synovial fluid studies at our institution to better define this degree of overlap and characterize the features of infectious arthritis in relationship to synovial fluid white cell counts. The records of 202 consecutive patients with synovial fluid white blood cell counts >2000/mm were reviewed. Infectious arthritis was diagnosed in 77% (10/13) of patients with counts >100,000, 47% (8/17) in the 50,000-100,000 range, and 5% (9/172) with counts <50,000. Crystal-induced arthritis and rheumatoid arthritis made up 81% of patients in the 15,000-50,000 range. Overall, 10 of 27 (37%) cases of infectious arthritis had white cell counts >100,000, and 18 of 27 (67%) had counts >50,000. A majority of these infections (14/18) were related to Staphylococcus aureus, while 5 of 7 infections associated with counts <20,000 were associated with atypical organisms. This study confirms that a majority of patients with very high synovial fluid white blood cell counts have infectious arthritis, and that the likelihood of infection is markedly reduced, but certainly not excluded, below this level. The presence of atypical infections in a small percentage of patients with low counts emphasizes the importance of clinical judgment in evaluating all patients with inflammatory arthritis, regardless of synovial fluid white cell counts.