BACKGROUND Irinotecan is currently used as second-line chemotherapy for advanced colorectal cancer. We report a case of severe thrombocytopenia after Irinotecan, suggesting an immune mechanism, in a 53-year-old patient. METHODS The patient's sera were screened for platelet antibodies with an indirect platelet immunofluorescence test (PIIFT). The monoclonal antibody immobilization of platelet antigen assay (MAIPA) was used to characterize the antibody target. RESULTS We detected an IgG platelet antibody in the patient's serum in the presence of Irinotecan by means of PIIFT, and not in the presence of SN-38, its active metabolite. The specificity of the binding was asserted after CD32 MoAb blockade. The platelet binding site could not be strictly identified with MAIPA and immunoblotting but GpIIb/IIIa can be excluded after experiments with Glanzmann platelets. CONCLUSIONS This case can be considered the first documented Irinotecan-induced immune thrombocytopenia.