The California Irrigation Management Information System (CIMIS) uses a modified Penman method for calculating reference ETo known as Pruitt-Doorenbos (PD). The CIMIS weather station in Five Points, CA is located on fescue grass cut to 0.1 m. A weighing lysimeter located 20 m from the weather station collects hourly weight data. Our objective was a comparison of PD predictions with evapotranspiration rates calculated from lysimeter data. After correcting the weight data for drainage a 7-point Savitsky-Golay filter computed the derivative at the central point. The data set for calendar years 2004-05 was culled removing entries for days when data were missing, corrupted or rainfall occurred using an interactive computer program developed for this purpose. The remaining hourly evapotranspiration data plotted against CIMIS hourly ETo predictions almost matched a 1:1 line indicating that the CIMIS predictions were accurate for the large data set. However, the scatter was substantial leading to a more detailed analysis to characterize sources of variability. The data were selected by hour and a best-fit line was calculated assuming a zero intercept. Slope of best-fit lines to CIMIS ETo vs. lysimeter ETo for data representing each hour from 9 AM through 2 PM the CIMIS ETo were consistently greater than one but consistently less than one at other times during the day. At night, correlation of hourly CIMIS and lysimeter ETo was insignificant (r²<0.2). These results suggest that CIMIS hourly ETo predictions at this site could be improved by characterization of factors that would improve prediction of ETo at night. Also, the further analysis of daytime CIMIS hourly ETo could provide an insight into how the calculation might be adjusted to provide a more accurate representation throughout the day.