||Gray leaf spot of maize (Zea mays L.), caused by the fungus Cercospora zeae maydis Tehon & Daniels, has become an economically important disease in tropical humid maize growing areas of the Republic of Colombia. To implement an adequate program of selection and improvement of genetic tolerance to this disease, it was necesary to determine the inheritance of this tolerance. For this purpose, five highly tolerant and five-susceptible maize inbred lines were evaluated using a diallel design (Griffing Model 1), at the Centro Internacional de Agricultura Tropical (CIAT), Colombia. Parental lines and their crosses, both direct and reciprocal, were evaluated in three different environments using conservation tillage and were artificially inoculated using a mixture of gray leaf spot infected leaves collected from farmers’ fields. The variable Disease Index (DI; scale 1: resistant; 5: highly susceptible) of the parental lines and their direct and reciprocal crosses was determined in three environments, in fields planted under direct planting, artificially inoculated with inoculum prepared using leaves with symptoms of the disease collected from farmers’ fields. In the combined analysis, there were highly significant differences for parents, crosses (direct and reciprocal), general combining ability (GCA) and specific combining ability (SCA). There were no significant differences for either parents vs. crosses or for direct vs. reciprocal crosses. Consequently, heterosis was not important in DI (mean for parents: 2.97, and for crosses: 3.03) and no maternal effects were observed for these lines (mean for direct and reciprocal crosses: 3.03). GCA sum of squares (SS) was three fold of the SCA SS, meaning that additive gene effects were more important than non-additive gene effects. No parent×environment interaction was observed. Interaction of environment with crosses, GCA, and SCA were highly significant.