Progression to androgen independence remains the main obstacle to improving survival and quality of life in patients with advanced prostate cancer. Induction of differentiation may serve as a rational basis for prevention of progression to androgen independence by modulating gene expression activated by castration or upregulated during androgen-independent progression. The objectives of this study were to characterize the in vitro effects of sodium butyrate on human prostate cancer cell growth, PSA gene expression, and differentiation in the LNCaP tumor model and to determine whether tumor progression in vivo is delayed by isobutyramide, an orally bioavailable butyrate analogue with a longer half-life. The effects of isobutyramide on LNCaP tumor growth and serum PSA levels in both intact and castrate male mice were compared to controls. At concentrations >1 mM, butyrate induced dose-dependent changes towards a more differentiated phenotype, G1 cell cycle arrest, and an 80% decrease in LNCaP cell growth rates. PSA gene expression was increased threefold by butyrate, indicative of differentiation-enhanced gene expression. The half-life of isobutyramide in athymic mice was determined by gas chromatography to be 4 h. During a 4 week period in intact-placebo mice, tumor volume and serum PSA increased 4.1- and 6.6-fold, respectively, compared to twofold and 2.7-fold increases in tumor volume and serum PSA in intact-treated mice. During a 7 week period in castrate-placebo mice, tumor volume and serum PSA levels increased 2.4-fold and fourfold, respectively, compared to a 50% reduction in tumor volume and a twofold increase in serum PSA above nadir levels in castrate mice treated with adjuvant isobutyramide. Isobutyramide treatment induced pronounced morphological changes in LNCaP tumor cells, with loss of defined nucleoli and dispersion of chromatin distribution. LNCaP tumor PSA mRNA levels actually increased threefold, indicative of differentiation-enhanced gene expression. This study demonstrates that butyrate causes LNCaP cell cycle arrest and increased PSA gene expression, both indicative of differentiation. The combination of castration and adjuvant isobutyramide was synergistic in delaying tumor progression. Decreased tumor cell proliferation and increased PSA gene expression induced by isobutyramide results in disconcordant changes in serum PSA and tumor volume and reduces the utility of serum PSA as a marker of response to therapy.