Three classic-type, small cell lung cancer cell lines (GLC-14, GLC-16, and GLC-19) have been established from one patient during longitudinal follow-up. During this period the tumor changed from sensitive to completely resistant to (chemo)therapy. A phenotypical and functional characterization of the different cell lines is given in combination with the matching clinical data. (a) The cell lines have been compared with the biopsies from which they were derived. There was a good match between the morphological, biochemical, and immunohistological findings in the cell lines as compared to those obtained in the biopsies. When the biopsy and cell line (GLC-14) obtained before the start of therapy were compared to the biopsies and cell lines (GLC-16 and GLC-19) acquired after the first and second reinduction therapy, respectively, no major changes could be observed. The only clear alteration was the loss of a neuroendocrine antigen (defined by monoclonal antibody MOC-51) in the posttherapy specimens. (b) The doxorubicin, melphalan, and etoposide sensitivity in vitro reflected the clinically observed development of resistance to treatment. The cell line (GLC-14) established before the start of therapy was more sensitive than the lines (GLC-16 and GLC-19) obtained after treatment. It is concluded that the cell lines described in this paper represent a well-characterized in vitro model in which the development of drug resistance in small cell lung cancer can be studied.