The sensitivity of the standardized ASCE grass-reference Penman-Monteith evapotranspiration (ASCE-PM ETo) equation to climate variables in different regions has not yet been studied. Sensitivity analyses for the standardized daily form of the ASCE-PM equation were conducted on wind speed at 2m height (U2), maximum and minimum air temperatures (Tmax and Tmin), vapor pressure deficit (VPD), and solar radiation (Rs) in the following regions of the United States: semiarid (Scottsbluff, Nebraska, and Bushland, Texas), a Mediterranean-type climate (Santa Barbara, California), coastal humid (Fort Pierce, Florida), inland humid and semihumid (Rockport, Missouri, and Clay Center, Nebraska), and an island (Twitchell Island, California). The sensitivity coefficients were derived for each variable on a daily basis. In general, ETo was most sensitive to VPD at all locations, while sensitivity of ETo to the same variable showed significant variation from one location to another and at the same location within the year. After VPD, ETo was most sensitive to U2 in semiarid regions (Scottsbluff, Clay Center, and Bushland) during the summer months. The Rs was the dominant driving force of ETo at humid locations (Fort Pierce and Rockport) during the summer months. At Santa Barbara, the sensitivity of ETo to U2 was minimal during the summer months. At Bushland, Scottsbluff, and Twitchell Island, ETo was more sensitive to Tmax than Rs in summer months, whereas it was equally sensitive to Tmax and Rs at Clay Center. The ETo was not sensitive to Tmin at any of the locations. The change in ETo was linearly related to change in climate variables (with r2≥0.96 in most cases), with the exception of Tmin, at all sites. Increase in ETo with respect to increase in climate variable changed considerably by month. On an annual average, a 1°C increase in Tmax resulted in 0.11, 0.06, 0.16, 0.07, 0.11, 0.08, and 0.10mm increases in ETo at Scottsbluff, Santa Barbara, Bushland, Fort Pierce, Twitchell Island, Rockport, and Clay Center. A 1 m·s-1 increase in U2 resulted in 0.42, 0.18, 0.37, 0.28, 0.31, 0.20, and 0.26mm increases in ETo at the same locations. A unit increase in Tmax resulted in the largest increase in ETo at Bushland, and a unit increase in Rs caused the largest increases in ETo at Fort Pierce. A 1 MJm-2·d-1 increase in Rs resulted in 0.05, 0.08, 0.06, 0.11, 0.05, 0.06, and 0.06mm increases in ETo at the same locations. A 0.4kPa increase in VPD resulted in 1.13, 0.54, 1.29, 0.57, 1.04, 1.10, and 1.22mm increases in ETo at the same locations. The U2 had the most effect on ETo at Scottsbluff and Bushland, the two locations where dry and strong winds are common during the growing season. The sensitivity coefficient for Rs was higher during the summer months and lower during the winter months, and the opposite was observed for VPD (except for Twitchell Island). The decrease of the sensitivity coefficients for Rs corresponding to an increase in the sensitivity coefficient for VPD is due to a decrease in the energy term in favor of the increase in significance of the aerodynamic term of the standardized ASCE-PM equation in summer versus winter months. Because the ASCE-PM and the Food and Agriculture Organization paper number 56 Penman-Monteith (FAO56-PM) equations are identical when applied on a daily time step, the results of the sensitivity analyses and sensitivity coefficients of this study should be directly applicable to the FAO56-PM equation.