The two stereoisomers of retinoic acid (RA), all-trans and 9-cis-RA, are regulators of cell proliferation, differentiation and apoptosis. In this study, the aim was to evaluate the effects of all-trans-and 9-cis-RA on cell growth, proliferation, and on the induction of apoptosis in the human small cell lung carcinoma (SCLC) cell lines NCI-H82 and NCI-H209. The application of various concentrations of all-trans and 9-cis-RA were able to inhibit cell growth and proliferation. Moreover, 3H-thymidine incorporation was inhibited and the number of viable cells decreased, suggesting that all-trans-RA and 9-cis-RA can inhibit cell proliferation in a dose dependent manner. Morphological examinations (light, electron and fluorescence microscopy) demonstrated that both retinoids had profound effects on the induction of apoptosis. Our investigation also showed that, compared to all-trans-RA, 9-cis-RA is a stronger inducer for the inhibition of cell growth and proliferation and that it is more effective in the induction of apoptosis in small cell lung carcinoma cells in culture.