Histologically, lymphogranulomatosis X (LgrX) is a Morbus Hodgkin-like disease which until now has been considered as an abnormal immune reaction or a prelymphoma. Chromosome analyses showed that LgrX and angioimmunoblastic lymphadenopathy AILD) are characterized by chromosomal aberrations. Chromosome analyses of 18 cases of LgrX with sequential banding techniques showed abnormalities in 13 out of 18 cases. They were monoclonal in 7 cases. The most frequent abnormalities were trisomies of chromosomes Nos 3 and 5 and duplication of the X-chromosome. The abnormal karyotypes always appeared with normal mitosis in single or clonal cells. They were found in unstimulated and in PHA-stimulated cultures from lymph node and peripheral blood. Thus, it can be concluded that at least some cases of LgrX are monoclonal cell proliferations. An attempt is made to define the role of chromosomal abnormalities in the development of malignant lymphomas.