Optical parameters of NaY(WO4)2 crystals were calculated from absorption spectra with Judd-Ofelt theory. Intensive infrared-to-visible frequency upconversion has been discovered and investigated under a 974 nm CW laser diode excitation at room temperature. The bright blue upconversion fluorescence emission is analysed in detail. The optimal concentration of Tm3+, which is about 1 at% for a Yb3+ concentration of 10 at% for the blue emission in the crystals, is much higher than that in other hosts. The experimental results and theoretical analysis suggest that NaY(WO4)2 with high phonon energy is most probably an even more promising crystal than fluoride. © 2006 IOP Publishing Ltd.