Cytogenetic analysis of short-term cultures from 26 primary bronchial large cell carcinomas and 1 metastasis from a primary large cell carcinoma revealed clonal chromosome abnormalities in 20 tumors and a normal karyotype in 6. No outgrowth was obtained in 1 case. Simple aberrations were present in 3 tumors; in 1, the only change was a reciprocal translocation between chromosomes 1 and 6, in another the sole anomaly was a supernumerary marker ring chromosome and in a third loss of the Y chromosome was the only clonal change. The remaining 17 tumors had complex karyotypes. The chromosomes most frequently involved in structural rearrangements were chromosomes 1, 2, 3, 5, 6, 7, 8, 10, 11, 13, 14 and 17. The bands most frequently affected were 1q11-12, 1p13, 7q11, and 17p11-13. The rearrangements led to repeated losses of 1p, 1q, 3p, 6q, 7q and 17p and gains of 5q and 7p. The emerging karyotypic picture of large cell lung carcinomas indicates more similarities with adenocarcinomas than with other pathologic subgroups of lung cancer.