Objectives. We present the first evidence-based method for estimating public health and cost impacts of partner notification (PN) that takes account of sexual partnership type. Methods. Our algorithm uses routine clinical data, probability survey data, and transmission parameters. We propose 2 new epidemiological concepts to quantify PN impact: "[the] absolute reduction in onward transmission" and its reciprocal, "[the] number needed to treat to interrupt transmission" (i.e., the number of partners who need to be treated to interrupt 1 onward transmission). We demonstrate these concepts for 273 chlamydia cases diagnosed at a UK genitourinary medicine clinic. Results. The number needed to treat to interrupt transmission (overall, for casual partners, and for regular partners, respectively) was 1.47, 1.11, and 2.50, respectively, for men younger than 25 years; 1.60, 0.83, and 1.25, respectively, for women younger than 25 years; 2.35, 1.39, and 2.08, respectively, for men older than 25 years; and 2.14, 0.93, and 2.08, respectively, for women older than 25 years. Conclusions. PN that targets casual partners, rather than regular or live-in partners, prevents more secondary transmissions per partnership; it is also more resource intensive, but the public health benefit is greater.