To identify the critical vector density that potentiates dengue outbreaks in an endemic site and to identify obstacles to anti-dengue activities, we correlated a series of dengue outbreaks in a Brazilian city with the intensity of its anti-vector source-reduction activities. The proportion of houses infested by vector mosquitoes correlated inversely with intensity of anti-mosquito interventions, and the vector population developed independently of rainfall. Local periods of drought promoted vector abundance in two ways: residents stored water in which vector mosquitoes could breed, and cholera outbreaks due to contaminated water diverted local health workers from routine anti-vector activities. One dengue outbreak became apparent to authorities more than two months after it commenced but would have been identified almost immediately had dengue-like disease in indicator hospitals been monitored. Active surveillance, therefore, offers a window of opportunity for promptly executed anti-dengue interventions. Source-reduction measures that suppress vector infestations to less than 1% of houses effectively avert outbreaks of dengue.