We study the magnetic field and the umbral intensity of sunspots for the period 1999 to 2011. We analyze full Stokes spectra of 183 spots observed with the Tenerife Infrared Polarimeter at the German Vacuum Tower Telescope on Tenerife. We derive the magnetic field strength in the umbra from the Zeeman splitting of a near-infrared spectral line. This procedure eliminates the influence of non-magnetic stray light from the spot surroundings. The systematic decrease of umbral magnetic field strength observed during the declining phase of cycle 23 does not continue into cycle 24, instead, we observe a significant increase of magnetic field strength in spots of the new cycle. This indicates that the observed variations of the magnetic field strength are dominated by a cyclic effect rather than by a long-term trend.