Reference evapotranspiration (ETo) is an important agrometeorological parameter for climatological and hydrological studies, as well as for irrigation planning and management. The FAO-56 Penman-Monteith combination equation (FAO-56 PM) has been recommended by the Food and Agriculture Organization of the United Nations (FAO) as the standard equation for estimating reference evapotranspiration (ETO). This method considers many parameters related to the evapotranspiration process; net radiation (Rn), air temperature (T), vapor pressure deficit (VPD), and wind speed (U). This paper examines the potential of FAO-56 PM equation in estimating the ETO under difference conditions from limited weather data. For this study, full weather data sets were collected from nine difference weather stations from Iran. FAO-56 reduced-set PM ETO estimates were in closest agreement with FAO-56 full set PM ETO estimates at the most of locations. The difference between FAO-56 full set PM ETO estimates and FAO-56 PM reduced-set ETO estimates generally increases by increasing the number of estimated weather parameters. Overall results indicate that FAO-56 reduced-set PM approaches mostly provided better results compared to Turc equation, adjusted Hargreaves equation and artificial neural networks. This fact strongly supports using the FAO-56 PM equation even in the absence of the complete weather data set. The minimum and maximum air temperature data and local default wind speed value are the minimum data requirements necessary to successfully use the FAO-56 PM equation under humid and arid and semi-arid conditions and the minimum and maximum air temperature data daily and wind speed data daily are the minimum data requirements under extremely arid conditions.