The procedure followed by the Nuclear Metrology Laboratory (LMN) at the IPEN-CNEN/SP, in S~ao Paulo, for the
primary standardization of 166Ho is described. The activity of 166Ho was determined by the efficiency extrapolation
technique applied to a 4πβ(PC)-γ coincidence system using a gas flow proportional counter in 4π geometry
coupled to a 76 x 76 mm NaI(Tl) crystal. The results for the γ-rays intensities at 80.57 and 1379.45 keV
were 0.0651(11) and 0.00904(11), respectively.