Under rigorous statistical controls, it has been shown that the larger the proportion of lower SES women enrolled in organized family planning programs, the lower their fertility. Program effects independent of other social, economic and cultural factors were shown for lower SES whites and blacks, and for most age groups. The potential of a fully implemented program to reduce fertility differentials between upper and lower SES groups was assessed, using 1969-1970 fertility rates and the estimates of 1969 program impact. Although we believe that the program's impact has increased in magnitude over time, even these estimates from an early point in U.S. program development provide impressive documentation that the program reduces fertility in the subpopulation served by the program, and, by implication, that there is a genuine need for organized family planning services, even in an industrialized nation like the United States. If there were no need, there could be no program effect. The family planning program was one of the major new health and social programs introduced in the mid-1960s. This study shows that, far from failing, the program was succeeding very well in attaining its objectives. The program works because it gives women of lower socioeconomic status access to modern and effective methods of contraception that they would not otherwise have. As a result, the rates of unwanted and mistimed pregnancy of patients are lower than those of comparable women who lack access to organized clinic programs.