The role that the urban environment plays in influencing drug users' injection and needle disposal decisions is poorly understood. We identified potential attractors and deterrents of needle discarding, and then used a geographic information system (GIS) to quantify these factors for a neighborhood in Montréal, Canada. In multivariate logistic regression, discarded needles were found to have more associations with physical factors than with social factors. Visual exposure and proximity to a single-room occupancy hotel, a pay phone, an adult service or a pawnshop were important physical environmental predictors. These findings are discussed in relation to developing public health and urban design-based harm reduction approaches to needle discarding in public space.