A simple and highly efficient method has been developed for the synthesis of 3,3-diindolyl oxyindoles by the reaction of indoles with isatin or 5-fluoro isatin using a catalytic amount (5 mol%) of FeCl(3) at room temperature in a short reaction time in high yields. All these compounds were evaluated against a panel of five human cancer lines and most of them showed potent cytotoxicity. Compound 4b showed IC(50) of 4.7 and 5 microM against SK-N-SH and DU-145 cell lines, respectively, whereas 4c, 4d, 4f and 4k showed IC(50) of 2.2, 1.2, 3.6 and 3.6 microM, respectively, against DU-145 cell line. Interestingly, some of the compounds are selectively potent in prostate cancer (DU-145) with IC(50) values of 1.2-19.6 microM.