1. Seven cases of ruptured spleen as a complication of infectious mononucleosis are described and reference made to the 3 cases previously recorded in the literature.

2. It proved possible to make an objective histologic diagnosis of infectious mononucleosis from well prepared sections of the spleens. The diagnosis was based on: (a) a blurred architectural pattern due chiefly to large numbers of atypical lymphocytes diffused throughout the pulp and clumped in the blood sinuses; (b) small, poorly defined follicles, usually without germinal centers, in less than usual numbers per unit area; (c) cellular "infiltrates," composed largely of normal and atypical lymphocytes, in the capsule and trabeculae, in the adventitia of small intratrabecular arteries, and in the subintimal zone of collecting venous sinuses and intratrabecular veins; (d) swelling of the lining or attached cells of the blood sinuses.

3. "Infiltration" of the capsule and trabeculae reached considerable proportions, occasionally to the point of complete dissolution of these structures, and served as a predisposing cause of rupture. The same changes were noted in intact spleens from fatal cases of infectious mononucleosis.

4. The spleen in infectious mononucleosis was 3 to 4 times normal size and ruptured during the third or fourth week of the disease.

5. The importance of trivial injury as the exciting cause of so-called "spontaneous" rupture of the spleen has been emphasized. It is recommended that extreme caution be employed during attempted palpation of the spleen in a suspected case of the disease. When the diagnosis is obvious, splenic palpation may well be omitted.

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