The objective of this study was to identify and classify the type, size and frequency of structural defects occurring in concrete storm water, sanitary and combined sewers that have been in service for a long time. Another aim was to develop a method of classification of those sewers into five categories of failure probability. The paper presents results of non-destructive CCTV surveys of concrete sewers, which were conducted in different Polish cities. It also describes the causes of the defects that are related or unrelated to the type of flowing sewage. In the method proposed in the study, the matrix PS was developed. Elements of this matrix were determined. They contain lower boundary values that define the size of the analyzed defects for five categories of sewer failure probability. The proposed method allows classifying every analyzed sewer into one of five categories of sewer failure probability. The method can be used as a tool in sewer rehabilitation planning. The key conclusions highlight the causes of sewer failures, the scale of hazards and the benefits resulting from the application of the proposed method to prioritize analyzed sewers into one of five categories of failure probability.