Proftest SYKE carried out the proficiency test (PT) in cooperation with the Finnish Radiation and Nu-clear Safety Authority (STUK) for laboratories conducting radon measurements in ground water in May 2021. In total, 25 participants took part in the PT and three of them provided two sets of results. Two ground water samples were provided, of which one contained lower (GRn1; < 1000 Bq/l) and the other contained higher radon concentration (GRn2; 1000-8000 Bq/l). The median of the participant results was used as the assigned value for radon concentration. The evaluation of the results was based on z scores. In total 88 % of the results were satisfactory when total deviations of 20 % from the assigned value was accepted. Warm thanks to all the participants of this proficiency test!