A national campaign was carried out in Italy for the application of coincidence-summing correction in gamma-ray spectrometry. Twelve laboratories, belonging to the National Environmental Radioactivity Surveillance Network, took part in the campaign. They are equipped with gamma-ray spectrometry systems based on p- and n-type HPGe detectors with different relative efficiencies. A simplified procedure was used for coincidence-summing correction. This procedure requires a single-photon, single-nuclide source to measure the peak-to-total efficiency ratio at just one photon energy value. All the laboratories were given a 137Cs standard source for total efficiency determination, and a 134Cs source in the same geometry whose activity they had to determine. The results show the usefulness of the procedure. The average deviation of all the laboratory results from the ENEA-INMRI reference value was reduced from -12% before correction to +1% after correction. The paper gives a description of the different organisational aspects of the campaign, reports the results obtained and draws conclusions in which the gain in measurement accuracy is evaluated in the light of the effort required for each participant to perform the correction.