The chromosomes from three uterine tumours found in the same patient, two benign leiomyomas (L22 and L23) and a low-grade stroma cell sarcoma with leiomyomatous differentiation (L24), were studied by banding technique. L23 had an abnormal stemline distinguished by triosomy 12. The sarcoma showed a stemline with the same 7q-marker as L23 plus a marker del (12)(q13-24). A comparison with cytogenetically studied myomas collected from the literature showed (1) that there are at least three different recurrent, primary, gross chromosomal changes, viz. t(12;14)(q14-15;q23-24), t(1;2)(p36;p24) and del(7)(q22-31) and that there exist at least five further types of primary chromosomal deviations. The sarcoma showed aberrations identical or closely related to the recurrent structural deviations in myomas. These observations indicate a similar evolutionary pattern in benign and malignant leiomyomatous tumours.