Cytotoxic chemotherapy induces cell differentiation in small-cell lung carcinoma. Despite the high response rates resulting from chemotherapy, the majority of small-cell lung cancer (SCLC) patients relapse with chemoresistant tumors. To analyze the phenotypic changes that are precursors of chemoresistant status, and to investigate the role of chemotherapy in these changes, tumor samples from 20 patients, taken before chemotherapy (etoposide, doxorubicin, and cyclophosphamide) and again at the onset of chemoresistance (after at least three courses of chemotherapy), were compared. The histologic changes were minor in 10 of 20 patients, as shown by an increase in cell size; they were major in 10 of 20 patients, with the appearance of mixed composite tumors in which neuroendocrine (NE), epidermoid, and glandular components were mixed. Major changes correlated with a good response to chemotherapy (P = .001). Ultrastructural studies showed an increase in neurosecretory granules and desmosomes, and a high frequency of multidirectional differentiation (45%) when comparison was made with pretherapy samples (10%) (P less than .01). Immunohistochemical (IH) analysis showed an increase in cytokeratin (CK) expression in treated patients, with a different labeling pattern and the expression of higher molecular weight CK. The expression of NE lineage markers (Leu 19, Sy 38, SL 11-14) remained stable, while that of NE differentiation markers (Leu 7, chromogranin) increased in the treated patients. The neuron-specific enolase (NSE) activity remained stable in treated SCLC. Large cells with a more differentiated phenotype and proliferative capacity (as shown by Ki 67 labeling), appeared to be characteristic of treated and secondary chemoresistant SCLC. The acquisition of a more complex phenotype, which correlates with primary response to therapy, implies a drug-induced differentiation in SCLC. 