BACKGROUND: In the northeastern United States, tickborne diseases are a major public health concern. In controlled studies, a single springtime application of acaricide has been shown to kill between 68 and 100% of ticks. Although public health authorities recommend use of acaricides to control tick populations in yards, the effectiveness of these pesticides to prevent tick bites or human tickborne diseases is unknown. METHODS: We conducted a 2-year, randomized, double-blinded, placebo-controlled trial among 2,727 households in three Northeastern states. Households received a single springtime barrier application of bifenthrin or water according to recommended practices. Tick drags were conducted 3-4 weeks post-treatment on 10% of properties. Information on human-tick encounters and tickborne diseases was collected through monthly surveys; reports of illness were validated by medical record review. RESULTS: Although abundance of questing ticks was significantly lower (63%) on acaricide-treated properties, there was no difference between treatment groups in human-tick encounters, self-reported or medical record-validated tickborne diseases. CONCLUSIONS: Used as recommended, acaricide barrier sprays do not significantly reduce household risk of tick exposure or tickborne disease. Measures for preventing tickborne diseases should be evaluated against human outcomes to confirm effectiveness.