Reported autochthonous dengue fever transmission in the United States has been limited to 5 south Texas border counties since 1980. We conducted a cross-sectional serosurvey in Brownsville, Texas, and Matamoros, Tamaulipas, Mexico (n = 600), in 2004 to assess dengue seroprevalence. Recent dengue infection was detected in 2% (95% confidence interval [CI] 0.5%-3.5%) and 7.3% (95% CI 4.3%-10.3%) of residents in Brownsville and Matamoros, respectively. Past infection was detected in 40% (95% CI 34%-45%) of Brownsville residents and 78% (95% CI 74%-83%) of Matamoros residents. For recent infection, only weekly family income <or=$100 was a significant predictor (adjusted odds ratio 3.2, 95% CI 1.3-8.0). Risk factors that predicted past dengue infection were presence of larval habitat, absence of air-conditioning and street drainage, and weekly family income <or=$100. Mosquito larvae were present in 30% of households in both cities. Our results show that dengue fever is endemic in this area of the southern Texas-Mexico border.