A retrospective study of patients with culture-verified septic arthritis (n = 54) and polarizing microscopy verified crystal-associated arthritis (n = 34) was conducted with the objective to identify discriminating laboratory parameters in serum. Serum CRP levels (p = 0.002) and ESR (p = 0.03) were significantly higher on admission in patients with septic arthritis than in those with crystal-associated arthritis. The peripheral WBC counts did not differ between the two groups, nor did the lactoferrin or procalcitonin (PCT) levels. Serum TNFalpha concentrations on admission were higher in patients with septic arthritis than in those with crystal-associated arthritis (p = 0.0008). Significant differences were also found for IL-8 (p = 0.01) and G-CSF (p = 0.002), but not for IL-6 (p = 0.5). However, extensive overlap between the groups was present, resulting in low sensitivity, specificity and predictive value for each test. Determining serum levels of acute phase reactants, including cytokines, does not replace careful synovial fluid examination, including direct microscopy and cultivation.