The objective of our research was to evaluate the impact of spring snowmelt inflow occurring in separate sewer systems on the temperature of raw sewage discharged into selected treatment plants supporting up to 2000 PE. The results were expected to indicate the range of sewage undercooling due to inflow of the melting water and to evaluate how long this state persists. The research was carried out from 1 January 2010 to 30 April 2010 in five selected sewer systems. The obtained results showed significant influence of snowmelt inflow on sewage temperature in the sewer system. Inflow, which during snowmelt constitutes from 43 to 70% of daily discharge into treatment plants, caused the decrease of average daily sewage temperature in the sewer system: in Gdów by 2.0ºC, in Kostrze by 2.2ºC, in Sidzina by 3.0ºC, in Skotniki by 2.5ºC, and in Wadów by 2.0ºC. The lowest values of hourly temperature of raw sewage during melt-water inflow to the sewer system reached the following values: 5.4ºC in Gdów, 5.0ºC in Kostrze, 4.6ºC in Sidzina, and 5.0ºC in Skotniki and Wadów. At such low average daily sewage temperatures, biological processes of nitrogen compound removal will be inhibited or even stopped. The research showed that the decreased sewage temperature lasted for 16 days. In this period, insufficiently treated sewage may be dangerous to collector water. The presented results provide another argument for increasing efforts to find out the reasons for inflow to the sewer system.