Background and purpose: People in Taiwan often take diet pills not prescribed by a doctor to reduce body weight, and serious side effects may ensue. This is the first case of a phentermine-related stroke directly confirmed by brain MRI. Case description: A young healthy woman developed a brain stem infarct only a few days after taking diet pills not prescribed by a doctor. Phentermine was detected in those pills by chemical analysis. Conclusion: Short-term phentermine use was the probable cause of the cerebral infarct.