Rheumatoid factor (RF) testing by latex fixation in 8,287 outpatients yielded a sensitivity of 81.6% and 78.0% at titers of 1:20 and 1:80, respectively, and a specificity against noninflammatory rheumatic disorders (NIRD) of 96.6% and 97.9% and against NIRD plus inflammatory disorders of 95.2% and 96.8%, respectively. The predictive value of a positive test result at the clinic prevalence rate for rheumatoid arthritis (RA) (16.4%) was approximately 80%, and was 70% at 10% prevalence and 10% at 1% prevalence. No associations of RF with age or sex were found in non-RA patients. RF titers increased minimally with age in RA patients and were higher in men than in women. This study suggests that latex testing is far more specific than has been believed and that the titer is not spuriously increased with age.