A case of thrombocytopenia associated with cimetidine administration in a 65-year-old white female with recurrent adenocarcinoma is reported. Her platelet count fell from 273,000/mm3 to 35,000/mm3 after cimetidine therapy was started. Upon discontinuation of cimetidine, the platelet count increased to 164,000/mm3. Later in her hospital course, the patient again received cimetidine and her platelet count fell to 37,000/mm3. The patient was never leukopenic during her hospitalization. This is the only reported case of thrombocytopenia in the absence of leukopenia that is documented by a rechallenge with cimetidine.