The seriousness of a disease is reflected in the amount of yield reduction caused by the disease under field conditions. We wished to obtain data on the amount of yield loss of corn (Zea mays L.) caused by the corn stunt disease complex which includes the maize chlorotic dwarf virus (also referred to as Ohio corn stunt agent) and the Mississippi corn stunt agent, believed to be a mycoplasma; and to determine if time of initial disease symptoms influences the amount of yield reduction. Corn hybrids were subjected to natural infection under field conditions during 5 years. Plants showing disease symptoms were tagged and yields from these plants were compared with plants not showing disease symptoms. Yields from diseased plants were less than 50% of those from healthy plants in each of the 5 years. The number of ears and kernels per plant was reduced more than was the weight of kernels on the diseased plants. Plants showing disease symptoms early in the season were more severely affected than plants that developed symptoms later. From our data, we concluded the following: diseased plants showing symptoms as early as 52 days after planting would produce no grain; plants showing first symptoms as late as 107 days after planting would produce normal yield; between the two extremes the yields on diseased plants would be reduced by 1.82% of that on healthy plants for each day the first symptoms appeared earlier than 107 days after planting.