[A case of supernumerary spleen in the left pleural cavity].

An extremely rare case of supernumerary spleen contained in the left pleural cavity is reported. Only one other case of this type is known. The anomaly, which was discovered by accident, may be explained by the probable delayed fusion of the mesenchymal tissue islands which form the spleen bud at the embryonic stage. One of these islands was imprisoned in the chest by the diaphragm, which was also in the final stages of development, thus creating the supernumerary spleen.