[en] Radioisotope techniques for measurement of the flow rate in the sewage central pipeline of the major area of Athens have been used and the eventual correlation of the flow rate to the sewage water level at the pipeline outlet in Keratsini coastal area has been investigated. The estimation of the flow rate by hydraulic calculations based on the sewage water level at the pipeline outlet in Keratsini coastal area was proved to be a not accurate method; this is attributed to the existing conditions at the outlet of the pipe, as well as to the extremely unstable variation rate of the flow rate. The actual relationship between the flow rate and the sewage water level at the outlet of the pipe was investigated and the inaccuracy accounted to the flow rate estimated by this relationship has been defined. (author)