A 48-year-old Caucasian woman presented a tender effusion of small volume, with a mild increase in skin temperature in her right knee and wide spread chondrocalcinosis found radiologically. She had normal renal function, had undergone a gastrectomy, and was receiving therapeutic doses of aluminum hydroxide antacids. Synovial fluid analysis showed small, irregular, nonbirefringent particles that stained intensely positive with the alizarin red stain. Scanning electron microscopy, energy dispersive X-ray analysis, and microprobe analysis showed aluminum and variable concentrations of chlorine and/or oxygen in isolated small spherical particles (average diameter about 15 micro). Neither phosphorus nor calcium was ever detected in the aluminum containing particles. These findings strongly suggest the presence of a mixture of aluminum hydroxide and some form of aluminum hydroxide chloride; neither was previously reported in synovial fluid of patients with normal renal function. Aluminum-containing particles should be considered an explanation for unexplained particles in joint fluid and may be a factor in some arthritis.