OBJECTIVE: To describe a case of thrombocytopenia associated with the administration of lansoprazole. CASE SUMMARY: An 85-year-old white man presented with an upper gastrointestinal hemorrhage from a gastric ulcer. His platelet count was normal on admission. He was started on oral lansoprazole 60 mg twice daily and, on hospital day 2, his platelet count decreased to 102 × 103/mm3; on hospital day 3, the platelet count was 36 × 103/mm3. Lansoprazole was discontinued, and the platelet count returned to normal. He has not had any further episodes of thrombocytopenia to date. DISCUSSION: After exclusion of other causes, the onset of thrombocytopenia after administration of lansoprazole, the resolution of the adverse reaction after discontinuation of the drug, and the fact that no other medicines were introduced during this time frame lead us to believe that this was most likely an idiosyncratic thrombocytopenic response to lansoprazole. To date, this is the first reported case of what appears to be isolated thrombocytopenia associated with lansoprazole. CONCLUSIONS: Clinicians should be aware of the possible association of thrombocytopenia with lansoprazole and discontinue the drug if thrombocytopenia becomes apparent.