The recently described B-cell lymphomas arising in mucosa-associated lymphoid tissue (MALT) form a distinct clinico-pathologic group of non-Hodgkin's lymphoma, and therefore would be expected to be characterized by a recurrent chromosomal aberration. We have analyzed the cytogenetics of 23 cases of MALT lymphomas arising in the stomach, small intestine, lung, and lacrimal gland. In each case the presence of an abnormal clonal cell population was confirmed by the identification of rearranged bands when digested tumor DNA was hybridized with a probe to the joining region of the immunoglobulin heavy chain gene. Metaphase spreads were obtained in 14 cases, of which 9 cases showed an abnormal karyotype. Although no unifying aberration was detected, rearrangements of chromosome 1p, and numerical abnormalities of chromosomes 3 and 7, may play a role in the genesis of these tumors.