This microarray dataset accompanies the publication by Kim et al, "The retinoic acid synthesis gene ALDH1a2 is a candidate tumor suppressor in prostate cancer". The dataset comprises four prostate cancer cell lines treated with the DNA methyltransferase inhibitor 5-aza-dC. For each cell line, gene expression in treated (channel 2; Cy5) and untreated (channel 1; Cy3) cells is directly compared by hybridization to the same DNA microarray. Further details on the treatment conditions are provided in the manuscript.