Pseudodiploid karyotypes with clonal structural rearrangements were observed in short-term cultures from two adenosquamous carcinomas (ASC) of the lung, a tumor type hitherto virtually uncharacterized cytogenetically. The karyotypes were 46,XX,der(1)t(1;14)(q44;q22),add(9)(q34) in the first tumor and 46,XX,i(17)(q10) in the second tumor. We suggest that ASC of the lung differ from other lung cancers by having pseudodiploid instead of massively aneuploid tumor stemlines.