Dwellings with/without a lead service line [LSL] were sampled for lead in tap water in Montreal, during different seasons. Short-term simulations using these results and the batchrun mode of the Integrated Exposure Uptake Biokinetic (IEUBK) model showed that children's exposure to lead at the tap in the presence of an LSL varies seasonally, and according to the type of dwelling. From July to March, for single-family homes, the estimated geometric mean [GM] blood lead level [BLL] decreased from 2.3-3.6 µg/dL to 1.5-2.5 µg/dL, depending on the children's age. The wide seasonal variations in lead exposure result in a minimal fraction (0-6%) of children with a predicted BLL >5 µg/dL in winter, as opposed to a significant proportion (5-25%) in summer. These estimations are in close agreement with the BLLs measured in Montreal children in fall and winter, and simulations using summer water lead levels illustrate the importance of measuring BLLs during the summer. Finally, simulations for wartime residences with long LSLs confirm the need to prioritize the control of this lead exposure from tap water.