Hydro power plant Pirot, which was built in 1990, is an accumulation-derivative power plant, which consists of 2 above-ground vertical hydroelectric generating sets that contain Francis turbines with nominal power of 41,5 MW, manufactured in Czech Republic, a tunnel and a sunken pipeline with overall length of 2.030 m and diameter that ranges from 3.000 to 3.500 m. Pipes have been made of S275J2G3 steel. Pipe wall is 22 mm thick. Maximum pressure of 2.5 MPa occurs in front of the turbine cover. Pipeline has been designed and built without anchor blocks at curvatures, which is a rarity elsewhere. Geodetic measurements have been conducted permanently from the day the assembly was finished and pipeline was put into service, both when pipeline is empty and unloaded by hydrostatic pressure and when it is full. Analysis of obtained data regarding the movements along the pipeline route showed that from year 2003 there are significantly higher differences in movements comparing the situations when the pipeline is full and when it is empty in comparison with the previous period. Those differences primarily refer to tangential movements of vertices marked with numbers 6, 7 and 8, which, compared to the period until year 2002, are in the range from 3 mm for vertex 8 to 5 mm for vertex 6. Apart from geodetic measurements, the measurement of pipe diameter in 2 directions is also being carried out permanently. Those data show that from year 2003 the diameter in horizontal direction started to increase significantly, while at the same time the diameter in vertical direction started to decrease less significantly. This paper contains the analysis of current state and integrity evaluation of the pipeline as a whole on the basis of results of non-destructive tests performed on the vital butt-welded joint in the curvature area at chainage 1+263 m (visual testing, magnetic particle testing, penetrant testing, ultrasonic testing, radiographic testing, metallographic replication testing.