We offer an economic perspective on prevention of β- thalassemia disease by means of genetic screening and prenatal diagnosis in an established program in Quebec province. The program screens 80% of at-risk persons in the high- risk communities, provides diagnosis to 75% of at-risk couples, and prevented two-thirds of new cases in the period of study. We measured the additional costs, in 1981 Canadian dollars, of medical and public health resources, both incurred and avoided, resulting from use of these prevention services. The total direct cost per case prevented in the program is less than the cost for a single year of treatment for an individual with the disease. Sensitivity analysis accommodating demographic assumptions, participation rates, and discounting rates indicates that, even at rates of marriage, endogamy, and participation lower than observed in the current program, treatment costs will still exceed prevention costs when discounting is set at conventional rates of 4% and 8%. Cost effectiveness of the program is confirmed.