This study assesses what physical and social factors increase the likelihood for fatalities during floods. The authors analyze 832 countywide floods in Texas from 1997-2001 with the aim of determining what factors characterize high percentages of casualties. Binomial regression models are used and adjustments are made by eliminating natural and built environment characteristics. Results show that the likelihood of a casualty caused by a flood increases with: flood duration, property damage, precipitation amounts on the day of the flood, population density, and most importantly for this study, the concentration of socially vulnerable people. Results also show that the likelihood decreases with: the level of precipitation on the day before the flood, the number of dams, and the level of preparedness among residents (mitigation strategies).