Fifty synovial fluid (SF) samples from patients with various types of arthritis were examined promptly after joint aspiration and after storage at room temperature (22 degrees C) or at refrigerator temperature (4 degrees C) for 1 hour, 2 hours, 3 hours, 6 hours, 1 day, and 3 days, then weekly for 3 weeks and monthly for 2 months. We found that the leukocyte count (white blood cell [WBC] count) decreased within a few hours. In 4 SF samples from patients with mild inflammation (initial range 3,150-6,200 WBC/mm3), the WBC count decreased into a "noninflammatory" range (less than 2,000/mm3) within 5-6 hours. In 3 of 5 SF samples that on the first day were found to be laden with crystals of calcium pyrophosphate dihydrate (CPPD), the crystals were much less abundant and were difficult to recognize by the next day. CPPD crystals dissolved completely in all SF samples by 3-8 weeks of the study. Monosodium urate crystals remained detectable throughout the 8 weeks of study, but they became smaller, less birefringent, and less numerous with time. Clumps of apatite-like crystals persisted for several months. Most SF samples initially negative for apatite-like crystals remained negative over time. New, artifactual crystals, including alizarin red S-positive clumps or star-shaped arrays, plate-like structures, positively birefringent Maltese crosses, and hematoidin crystals, developed with time. Because of these observations, we urge prompt examination of SF specimens to avoid the problems of misdiagnosing borderline inflammatory fluids, missing CPPD crystals that dissolve with time, or over-interpreting the findings because of the new, artifactual crystals.