This study reports on the evaluation of the feasibility, cost, and cost-effectiveness of a proactively provided telephone-based motivational smoking cessation intervention to an underserved population of pregnant smokers who may or may not receive ongoing prenatal care. As part of the New England SCRIPT randomized clinical trial comparing the efficacy of three types of smoking cessation interventions for pregnant smokers, one-third of the women (n=358) received a motivational telephone counseling intervention (MI) delivered by trained counselors using a semistructured protocol. Although this population was very mobile, the MI counselors were able to reach 86% of the women with at least one call and 46% received all three calls. The group receiving three MI calls had a cotinine-confirmed quit rate of 23%. Cost-effectiveness analyses for those women receiving telephone counseling supported the net benefit in favor of the three phone calls compared with the women who did not receive any telephone calls, with an effectiveness to cost ratio of 1:$84. Our results suggest that telephone-based motivational smoking cessation counseling may be a feasible and cost-effective method for low-income pregnant smokers enrolled in prenatal care.