Several northern hemisphere stratospheric meteorological data sets are shown to contain stationary anomalies. We consider four possible explanations for the anomalies: 1) real stationary-wave features; 2) biases in the analysis and assimilation methods; 3) errors in data input into the analysis and assimilation systems; and 4) tidal signals that are undersampled in the daily analyses. Because the easterly flow in the summer stratosphere is not consistent with stationary waves and the anomalies are present in multiple analyses, we conclude that the anomalies are a combination of biases in the input data and tidal signals that are aliased to zero frequency by daily sampling. Because the anomalies are large, they can have significant impacts on many applications of the data, including trajectories and chemical transport calculations. ¿ 1998 American Geophysical Union