Highly water-soluble ultrathin and fluorescent CdS nanorods are directly synthesized with the assistance of PEI. PEI plays as the passivator to control and induce the growth of the CdS nanorods and also makes the as-obtained nanorods dispersible in water. The as-obtained CdS nanorods are quite thin thus showing the quantum effect in the fluorescence. By adjusting the alkalinity of the solution, CdS nanorods can be controlled to precipitate or redisperse in water, which is believed to stem from the variation of the charges of the capping PEI molecules. These results provide an efficient and convenient way to directly synthesize water-soluble and fluorescent nanocrystals, which has potential application in biological labeling and detection.