Septic arthritis at the site of previous arthroplasty is a serious medical problem with high morbidity and mortality. Even with careful physical examination and laboratory evaluation, the diagnosis of septic arthritis may be difficult to confirm and many patients undergo operative procedures for presumed infection. In patients with previous arthroplasty, a synovial fluid white blood cell count of >2,500-3,000/mm 3 in the presence of an elevated erythrocyte sedimentation rate and C-reactive protein has been reported to indicate infection with a high degree of sensitivity and specificity. However, crystalinduced disease (gout, pseudogout) may present with the extact same clinical manifestations, physical examination, and laboratory results as infection. Yet crystal-induced disease is only rarely recognized in patients with previous arthroplasty. We report two cases of acute crystal disease in patients with previous arthroplasty that mimicked infection. Review of the few additional case reports extent suggests that crystal-induced disease may occur more frequently than heretofore recognized. We therefore propose that the evaluation of acute arthritis in a patient with previous arthroplasty include systematic evaluation for crystal disease, as patients may present with septic arthritis, crystal disease, or both.