Calculation of the high-latitude distribution of the vertical total electron content (TEC) is possible using a three-dimensional, time-dependent ionospheric model. Global and local comparisons may be made with observations of TEC. We compare the local diurnal variation of TEC calculated by the model with observations of TEC at Goose Bay, Labrador and Hamilton, Massachusetts. Data from the period of March 1–11, 1989, and monthly averaged data for solar maximum and solar minimum periods are examined. We extend the model to predict diurnal variations of TEC in the polar cap and compare these results with the observed TEC at Thule, Greenland, during an 8-day campaign from January 28 through February 4,1984. We propose a possible explanation for the large variability observed. We show that the “equivalent vertical content” TEC is very sensitive to horizontal F layer electron density gradients and that such “equivalent vertical” TECs may vary significantly from the true vertical TEC of the ionosphere. By incorporating these results, we calculate the vertical TEC distribution of the high-latitude ionosphere for a wide range of solar activity, seasons, and Kp variation represented by a recently completed Utah State University time-dependent ionospheric model data base. Finally, we discuss the possible uses of TEC as a diagnostic tool for testing ionospheric models.