Background: The relative effectiveness of in-person versus telephone interviews for human immunodeficiency virus (HIV)/sexually transmitted disease partner services (PS) is uncertain. Methods:We compared outcomes of in-person versus telephone PS interviews for early syphilis (ES) and newly diagnosed HIV in King County, Washington from 2010 to 2014. We used multivariable Poisson regression to evaluate indices (number of partners per original patient [OP]) for partners named, notified, tested, diagnosed, and treated (ES only). Analyses controlled for OP age, sex, race/ethnicity, sexual orientation, time to interview, place of diagnosis, and staff performing interviews. Results: For ES, 682 and 646 OPs underwent in-person and telephone interviews, respectively. In-person syphilis PSwere associated with higher indices of partners named (in-person index [IPI], 3.43; telephone index [TI], 2.06; adjusted relative risk [aRR], 1.68; 95% confidence interval [CI], 1.55-1.82), notified (IPI, 1.70; TI, 1.13; aRR, 1.39; 95% CI, 1.24-1.56), tested (IPI, 1.15; TI, 0.72; aRR, 1.34; 95% CI, 1.16-1.54), and empirically treated (IPI, 1.03; TI, 0.74; aRR, 1.19; 95% CI, 1.03-1.37), but no difference in infected partners treated (IPI, 0.28; TI, 0.24; aRR, 0.93; 95% CI, 0.72-1.21). For HIV, 358 and 489 OPs underwent in-person and telephone interviews, respectively. In-person HIV PS were associated with higher indices of partners named (IPI, 1.87; TI, 1.28; aRR, 1.38; 95% CI, 1.18-1.62), notified (IPI, 1.38; TI, 0.92; aRR, 1.24; 95%CI, 1.03-1.50), and newly diagnosed with HIV (IPI, 0.10; TI, 0.05; aRR, 2.17; 95% CI, 1.04-4.50), but no difference in partners tested (IPI, 0.61; TI, 0.48; aRR, 1.15; 95% CI, 0.88-1.52). Conclusions: Although in-person syphilis PS were associated with some increased PS indices, they did not increase the treatment of infected partners. In contrast, in-person HIV PS resulted in increased HIV case finding. These data support prioritizing in-person PS for HIV and suggest that inperson PS for syphilis may not have major public health benefit.