Using data from terrestrial global positioning system (GPS) receivers, we describe an anomalous drift in columnar water vapor measurements from the TOPEX microwave radiometer (TMR). Based on their long-occupation histories and proximity to open-water TOPEX/POSEIDON (T/P) ground tracks, we identified four geodetic-quality GPS stations that are suitable for calibrating the TMR. We constructed time series at each site based on the differences of instantaneous vertical wet tropospheric path delay (PDwet) derived independently from GPS and TMR data at T/P overflight times. The ensemble results span 1992--1997 and suggest that the TMR measurements of PDwet are gradually drifting lower. Our nominal estimate of the drift is -1.2¿0.4 mm yr-1 (one standard error). Accounting for this would increase the estimated rate of change in global mean sea level from T/P by the same amount. ¿ 1998 American Geophysical Union