This report describes the unusual case of a patient affected by Crohn's disease suffering from intestinal obstruction with recurrent occlusive symptoms not due to the intestinal disease, but to the presence of two calcified foreign bodies in the pelvis. The stones were surgically removed and analysed by reverse-phase liquid chromatography coupled to UV diode array detection and mass spectrometry (LC-UV-DAD-MS/MS), Chromatoprobe-MS/MS and by Fourier-transform-infrared spectroscopy (FT-IR) techniques. The combined mass spectrometric approaches allowed unequivocally to identify 5-aminosalicylic acid (5-ASA) in stone 1, and to demonstrate that its formation was due to an unmodified 5-ASA tablet, a formulation that must undergo complete dissolution in the small bowel. The second stone was constituted by a solid layer (no solvent-extractable material) identified by FT-IR as a polystyrene fragment. This indicates that accidental ingestion of a plastic material, followed by its calcification, was responsible for its formation.