A case of mesothelial cyst of the right hemidiaphragm in a 44 year old man is reported. Origin (most likely due to a recess of the primary coelomiatic cavity), topographical, histological and therapeutic features are discussed, stressing that it is now more frequently recognised thanks to progress in diagnostic imaging techniques. Complete surgical excision is mandatory to definitely differentiate such lesion from primary diaphragmatic neoplasm, not infrequently malignant.