A controlled prospective study of an infant immunization surveillance program in Rhode Island demonstrated significant motivational effects of the program on parents of newborn infants. Immunizations were initiated more often at the earliest appropriate age and higher overall levels of immunization with certain vaccines were obtained in the portion of the population subjected to the program than among controls. The effect was most pronounced in the lowest social class and diminished successively with increasing social rank. Costs were estimated to be $42 per additional child immunized. Strengths and weaknesses revealed by the study suggest that costs could be further reduced by nearly one third with but minor changes in the program, and without diminishing its effectiveness.