During periods of rain, combined sewer systems carry urban runoff water to sewage works. Urban runoff water is different, however, from household wastewater — the usual influent of wastewater treatment systems — especially the flow and the concentrations of suspended solids and metals. The purpose of the research described herein was to examine potential effects of runoff water on a treatment system to find out if major damage to the system is likely to result from input of urban runoff. The literature on urban runoff and on qualitative, toxic, quantitative, and hydraulic shocks was reviewed, and the knowledge gained was applied to a case study based on real data collected from metropolitan Quebec City (Canada) in June 1985. Experimental data indicate that the recorded variations in flow and in quantities of suspended solids, organic matter, and nutrients in the combined system following various rain events would not dramatically affect operation of a wastewater treatment system. The same conclusion applies for the increase in concentrations of copper and zinc. Further research on qualitative shock and the impact of the increase in the concentration of lead is required. Key words: biological treatment, waste waters, quality, concentration, rainfall, urban runoff waters, shock loads, toxics. [Journal translation]