An early event in the pathogenesis of neuroblastoma (NB), a tumor derived from embryonal neural crest tissue, appears to be the arrested differentiation of neuroblasts. However, NB cells can be induced to differentiate in vitro with numerous chemicals including retinoic acid (RA) and dibutyryl cyclic AMP (db-cAMP). One family of transcription factors, encoded by the homeobox (HOX) genes, plays a crucial role in Drosophila, Xenopus, and mammalian embryonic differentiation and development. We have previously identified six HOX genes (HOXC6, HOXC8, HOXD1, HOXD4, HOXD8, and HOXD9), by a sensitive PCR-based approach, in a cDNA library prepared from the human LA-N-5 NB cell line induced to differentiate with RA. In this report, we studied the regulation of these six HOX genes in a series of NB cell lines chemically induced to differentiate. Untreated NB cells express low or undetectable levels of HOX mRNA, and HOXC8 remains undetectable in the induced cells. However, a significant induction of HOXC6, HOXD1, and HOXD8 expression is seen in the RA-treated NB cell lines, albeit with different patterns and degree of up-regulation. db-cAMP treatment also induced HOXC6 and HOXD8 expression in two of the three NB cell lines analyzed. Low levels of HOXD4 and HOXD9 induction were observed in two and one RA-treated NB cell line, respectively. Up-regulation of HOXC6, HOXD1, and HOXD8 expression in human NB cells, chemically induced to differentiate, appears to be associated with maturation toward a differentiated neuronal phenotype.