We report a key comparison of 1 kg stainless steel mass standards carried out between February 1995 and October 1997. Organized within the Consultative Committee for Mass and Related Quantities (CCM), the participants included 14 national metrology institutes (NMIs) and the BIPM, with the BIPM serving as pilot laboratory. The 14 NMIs were divided into two groups, each of which determined the mass of two cylindrical travelling standards. In preliminary measurements, the pilot laboratory had determined the important physical properties of the four travelling standards and had monitored their mass stability over time. Measurements within the two groups then proceeded in parallel, with the four travelling standards returning several times to the pilot laboratory in order to monitor their stability. One of the present authors (MG) shows in appendix 3 how to treat comparisons among participants in different 'loops'. As shown in appendix 4, the mass reported by each NMI was derived indirectly from the mass of its national prototype, introducing a significant correction for air buoyancy during this step.The median of all results was taken to be the reference value for this key comparison. The degrees of equivalence are shown in tables 3–5 and summarized graphically in figure 7.Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The final report has been peer-reviewed and approved for publication by the CCM, according to the provisions of the Mutual Recognition Arrangement (MRA).