A statistical analysis of observations of large-scale undulations during the 23rd cycle of solar activity was performed using optical data from two stations: Tixie (71.6°N, 128.9°E) and Zhigansk (66.8°N, 123.4°E). The total number of events recorded was 54 (43 events at Tixie and 11 at Zhigansk). The complete list of observed events is presented. The occurrence frequency of eveningside (17–23 LT) undulations during the solar activity growth (1999) and decline (2003–2005) phases tends to increase. Large-scale undulations were shown to be generated both on the equatorward boundary of the diffuse auroral zone and inside the diffuse zone, which does not necessarily occur during magnetic storms.