Most previous cost-benefit analyses of fluoridation programs have been retrospective in approach and have biased their results in favor of fluoridation. This study performed a prospective cost-benefit analysis of fluoridating a segment of the water supply for Houston, Texas, and explicitly introduced and evaluated the time pattern of the costs and benefits. It was shown that neglect of the time structure of the costs and benefits would significantly bias the results. A benefit-cost ratio of 1.51 and a net present value (or "social profit") of $1,102,970 were found. The results are biased downwards and should be considered a lower bound. Thus the results indicate an investment in a fluoridation program by Houston would be a socially profitable one.