Radiation use efficiency (RUE) is an extremely useful ratio in crop productivity studies as it relates biomass production to solar radiation received by the crop. Cultivation of sweet sorghum For energy purposes has generated great interest in Europe, and RUE can be a valuable approach to sweet sorghum yield forecasting. The aim of this work was to determine the RUE of sweet sorghum cv. Keller grown in non-limiting water conditions in Madrid (central Spain). Productivity in aerial biomass and in sugars was studied during two crop cycles and the canopy radiation interception was determined. Yields were estimated at 2.67 10(3) g dry matter (DM) aerial biomass m(-2) and 835 g sugar m(-2), for the following crop conditions: 122 days after emergence (DAE) cycle length, 1350 thermal time, 597 MJ intercepted photosynthetically active radiation (PAR) m(-2). The canopy extinction coefficient was empirically determined as 0.57 for the interception efficiency. RUE, calculated as the slope of the regression of above-ground biomass and accumulated PAR(i), was estimated at 4.96 g DM MJ(-1). The energy efficiency of the crop, energy content of the total biomass per unit of incident global radiation, was 2.3% for the crop season and 1.1% for the full year.