In recent years pesticide vulnerability assessments have become increasingly important due to the increased detections of pesticides in groundwater. It is common to use simple vulnerability models that rank pesticides based on their leachability. A model sensitivity study was conducted using the leaching potential index model to evaluate the relative importance of input-parameter variability on the model predicted vulnerability. Parameters that were analyzed included organic-carbon content, depth to groundwater, soil-water velocity, soil bulk density, water content, organic-carbon partition coefficient, and pesticide degradation half-life. Results indicate that the calculated vulnerabilities are most sensitive to organic-carbon content, depth to groundwater, and vertical soil-water velocity. The sensitivity of the model to organic-carbon partition coefficient and biochemical half-life depends on the range of values given in the literature. Predicted vulnerabilities are not sensitive to bulk density and water content. This information should be useful to planners and regulatory officials who wish to determine the most important parameters to obtain measurements for when economic resources are limited.