Failure to follow up outpatient test results in a timely manner is a growing patient safety concern. To investigate the follow-up of markedly elevated serum potassium levels in the ambulatory setting, the authors reviewed the medical records of all patients seen in a large primary care practice between September 1, 2003, and August 31, 2004, with potassium levels > or = 5.8 mEq/L. Of the 12,914 serum potassium tests performed, there were 109 cases of markedly elevated serum potassium levels in 86 patients. The median potassium level was 5.9 mEq/L (range, 5.8-7.3). More than half the patients were recalled to the clinic specifically for repeat testing; however, 25% of patients had no repeat tests until they were seen at routine follow-up visits. The median time to a repeat potassium level was 6 days (range, 0-445). Patients > or = 65 years old had a lower likelihood of having repeat testing within 1 week (odds ratio = 0.38, P = .03).