In a prospective study of 36 patients (28 with olecranon and 8 with prepatellar bursitis) were recruited from 54 consequtive patients with these conditions. The bursitis was classified either septic or non-septic by an independent panel of senior clinicians, not otherwise involved in the study, on the basis of clinical, laboratory, treatment, and follow-up data. Median white cell counts in septic cases were higher than in non-septic cases (p<0.001, Mann-Whitney U test). Counts > 2000 x 10-6/l had a sensitivity of 94% (95% CI 81% to 107%) and a specificity of 79% (95% CI 60% to 98%) in the detection of septic cases.