166Ho (T1/2≈ 26.8 h) is an emerging theragnostic radionuclide of interest in nuclear medicine due to its peculiar decay scheme, featuring high-energy β− emission (≈ 1.8 MeV) coupled with the main gamma-ray emission (≈ 80.6 keV). Using the new 166Ho activity standard and the well-calibrated, high-energy resolution HPGe detector, both available at ENEA-INMRI, a new determination of several 166Ho gamma-ray emission intensities, Iγ, was performed with low uncertainty. The new Iγ values contributed to the Decay Data Evaluation Project. •166Ho and its use in nuclear medicine as theragnostic radionuclide.•Absolute activity measurements by Triple-to-Double Coincidence Ratio (TDCR) technique.•Hidex and portable ENEA-INMRI TDCR counters.•Gamma-ray spectrometry technique.•166Ho gamma-ray emission intensities.