To evaluate partner notification of opposite-sex sexual partners of AIDS patients as a means of limiting sexual and vertical transmission of human immunodeficiency virus (HIV), the authors examined the first 27 months of their experience with partner notification. Overall, of 145 AIDS patients eligible to participate, 51 (35%) were interviewed and identified 135 opposite-sex sexual partners. Of the 135 partners, 59 (44%) were interviewed and 34 (25%) were tested, resulting in the diagnosis of 7 (5%) HIV-infected partners. Refusal rates for index patients and partners were low (9% and 12%, respectively). Costs for the program were $454 per partner interviewed and $2,203 per seropositive partners identified. The authors conclude that although partner notification is more expensive than more widely targeted AIDS prevention and education efforts, its ability to target case finding, education, and counseling to women at highest risk of infection makes it potentially cost-effective for prevention of vertically transmitted HIV infection.