Aluminum toxicity can cause osteomalacia, anemia, and dementia in hemodialysis patients and has historically been associated with exposure to contaminated water or dialysate preparations or ingestion of aluminum-containing phosphate binders. Since 2002, improvements in water treatment methods and use of non-aluminum-containing phosphate binders have resulted in low prevalence (<1%) of aluminum toxicity among hemodialysis patients. In the United States, reported cases of aluminum toxicosis are rare, and no outbreak has been reported since 1992. This report describes 10 patients treated at a hemodialysis unit in a Wyoming hospital (hospital A) in 2007 who had elevated serum aluminum levels that were detected through routine serum aluminum screening. An investigation was conducted by the Wyoming Department of Health, which determined that the source of exposure was dialysate acid concentrate that became contaminated with aluminum as it passed through two electric drum pumps. The drum pumps had been used to transfer dialysate acid concentrate from 55-gallon storage drums to 1-gallon jugs for use on individual hemodialysis machines. Removal of the pumps from service resulted in a rapid reduction in patient serum aluminum levels. The findings suggest that regular assessment of machine compatibility with dialysate fluid is needed.