Context. According to theory, high-energy emission from the coronae of cool stars can severely erode the atmospheres of orbiting planets. No observational tests of the long-term erosion effects have been made yet. Aims: We analyze the current distribution of planetary mass with X-ray irradiation of the atmospheres to make an observational assessment of the consequences of erosion by coronal radiation. Methods: We studied a large sample of planet-hosting stars with XMM-Newton, Chandra, and ROSAT, carefully identified the X-ray counterparts, and fit their spectra to accurately measure the stellar X-ray flux. Results: The distribution of the planetary masses with X-ray flux suggests that erosion has taken place. Most surviving massive planets (Mp sin i > 1.5 MJ) have been exposed to lower accumulated irradiation. Heavy erosion during the initial stages of stellar evolution is followed by a phase of much weaker erosion. A line dividing these two phases could be present, showing a strong dependence on planet mass. Although a larger sample will be required to establish a well-defined erosion line, the distribution found is very suggestive. Conclusions: The distribution of planetary mass with X-ray flux is consistent with a scenario in which planet atmospheres have suffered the effects of erosion by coronal X-ray and EUV emission. The erosion line is an observational constraint for models of atmospheric erosion. Table 1 is only available in electronic form at http://www.aanda.org