Trials were planted at Rio Bravo, Tamaulipas, Mexico, during 2002 and 2003 with the objective of determining the relationship between sorghum ergot severity and weather factors, and to develop a risk assessment model. Six sorghum hybrids and three male-sterile genotypes were planted every month from January to October. At anthesis initiation, inoculation was conducted using a local isolate of Claviceps africana. Among hybrids, there was a negative relationship between ergot severity and maximum and minimum temperatures, with the highest significant correlation of –0.71 from 7 to 9 days before anthesis. During this period, minimum temperatures above 10°C increased the risk of ergot development, whereas minimum temperatures above 22.5°C prevented ergot development. In male-sterile plants, ergot was negatively related to maximum temperature after anthesis, with ergot being observed at maximum temperatures up to 38°C. Minimum relative humidity showed a positive and significant correlation with ergot severity. Values of minimum relative humidity above 30% during anthesis promoted infection. Surface response regression models were developed for the effects of minimum relative humidity and minimum and maximum temperature during the infection process on ergot severity.