A survey of 39 sanitary sewer collection systems was undertaken to determine the magnitude of hourly peak flows and to identify parameters most influencing the observed peaks. The significant parameters identified included: industrial contribution, collection system age, depth to groundwater and low lift pre-plant pumping stations. Collection systems with larger industrial contributions were observed to have higher peak flows than those with small industrial contributions. Variations in observed peak flows were exhibited between spring and summer periods in high groundwater and among old-age systems due to infiltration during the spring season. Low lift pre-plant pumping stations, depending upon their capacity and control, can create extremely high peak flows and pulses that do not reflect normal diurnal influent flow patterns.