Three inverted nasal papillomas were cytogenetically investigated after short-term culture. Two of the cases were characterized by a single abnormal clone with t(1;8)(p36;q11) and trisomy 7, respectively, whereas the third papilloma showed extreme cytogenetic heterogeneity: of 852 analyzed cells, 329 belonged to 36 unrelated clones, 344 had non-clonal changes, and 179 had a normal chromosome constitution. The polyclonal papilloma was further analyzed during in vitro passage of 3 lines (L1-L3) cultured independently since initiation of the primary cultures and found to have 6, 16 and 6 unrelated clones at analysis of primary cultures. At passage 1, each line was further subdivided into 2 sub-lines (L1A and B, L2A and B, and L3A and B), which were cultured separately until the cells spontaneously stopped dividing. After 4 to 7 passages, each sub-line was dominated (83-98% of the cells) by a single clone. The cell populations that took over the cultures were the same within each set of sub-lines (A and B lines), demonstrating that clonal overgrowth in vitro is not random. The difference in clonal selection among the L1-L3 lines further shows that genetic convergence during in vitro growth in stable conditions is dependent not only on the clones' ability to adapt to the culture conditions, but also on the nature of the neighboring cells with which they collaborate and compete.