Lung cancer is the major cause of cancer-related death in the United States, with small-cell lung cancer (SCLC) constituting approximately 20% of all cases of lung cancer. Numerous epidemiologic and molecular studies have suggested that alterations in retinoid-signaling pathways play a role in the pathogenesis of lung cancer. Fenretinide [ N -(4-hydroxyphenyl)retinamide; HPR] is a synthetic retinoid with minimal toxicity and favorable pharmacokinetics during long-term administration to patients in clinical trials. The aim of this investigation was to study the effect of HPR on the growth of SCLC cells in vitro. Seven SCLC cell lines (NCI-H69, NCI-H82, NCI-H146, NCI-H209, NCI-H345, NCI-H446, and NCI-H510A) were exposed continuously to a broad range of concentrations of HPR or all- trans -retinoic acid (RA), and cell viability was determined on day 3 and day 7 by the trypan blue dye exclusion assay. The growth of these cells was compared with that of control vehicle-treated cells to determine survival fraction and the dose resulting in a 50% inhibition of growth when compared with growth of control cells (IC 50 ). The induction of apoptosis was evaluated by fluorescent microscopy, DNA content analysis, and a terminal deoxyribonucleotidyl transferase-based assay that labels 3'-hydroxyl ends of DNA fragments (TUNEL assay) combined with flow cytometric analysis. HPR inhibited growth of a panel of SCLC cell lines at IC 50 values that ranged from 0.1 to 3.0 μ M (concentrations that are clinically achievable). In all cell lines tested, HPR was a more potent growth inhibitor than RA. By use of fluorescent microscopy, HPR was found to induce morphologic changes consistent with apoptosis in NCI-H82 SCLC cells, including cellular shrinkage, chromatin condensation, and nuclear fragmentation. Flow cytometric analysis revealed decreased DNA content, and TUNEL assay showed increased digoxigenin-uridine triphosphate incorporation in HPR-treated NCI-H82 SCLC cells; these findings are consistent with the induction of apoptosis. HPR inhibited the in vitro growth of SCLC cells. In NCI-H82 cells, HPR inhibited growth via the induction of apoptosis. [J Natl Cancer Inst 1995;87:1674–80]