Infectious mononucleosis is generally considered to be a benign disease with a clinical course, lasting from three to six weeks. However, in a small portion of these cases, serious complications may occur, one of these being spontaneous rupture of the spleen. The first case of spontaneous rupture of the spleen in infectious mononucleosis was reported by King, in 1941. Subsequently, there have been approximately 21 spontaneous or nearly spontaneous ruptures of the spleen associated with infectious mononucleosis reported, all being in males. There are at present an increasing number of infectious mononucleosis cases being diagnosed. It is, therefore, important to be aware of serious complications that may arise. Report of Case A 19-year-old white youth was admitted to the U. S. Naval Hospital, Pensacola, Fla., on Dec. 14, 1955. On admission cervical adenopathy with tenderness was present. The patient related that he had been in good health until Dec. 4,