Soybean seed is usually not carried-over to the next planting season because it deteriorates rapidly in storage, resulting in poor growth when planted. This rapid deterioration is a serious problem in seed production. The short shelf life of soybean seed is the result of its high lipid content and high levels of polyunsaturated linolenic and linoleic acids. The soybean production practice of planting early to maximize yield has led to routine use of seed treatments to protect the seeds and seedlings during the early stages of development. The amount of treated seed is increasing every year as research identifies more effective active ingredients. However, excess treated soybean seed must be disposed of differently from untreated seed, which generates an additional cost for the seed industry. There is a need for prolonging storage life of carry-over treated seeds to minimize seed disposal costs. The objectives were to determine the best storage conditions of temperature and relative humidity that will minimize the deterioration of chemically treated soybean seed from different maturity groups and seed composition. Twenty-four soybean varieties were treated with the fungicides fludioxonil and mefenoxam, or a mixture of these fungicides and the insecticide thiamethoxam, or left untreated as a control. The seeds were packaged and stored under one of three storage conditions: a non-climate controlled warehouse, a climate controlled coldroom (10^°C and 59.6±7.3% RH), or a climate controlled warmroom (25^°C and 31.2±11.1% RH). The decline in viability and vigor was evaluated at 4, 8, 12, 16 and 20 mo after storage. After 20 mo in storage, the mean moisture content of seed lots in the coldroom ranged between 10.15 to 10.77%, in the warmroom ranged between 5.66 to 5.81 %. The moisture contents for seeds in the warehouse ranged between 11.4 and 12.7%. Soybean genotypes differed significantly in their rate of decline of seed viability and vigor over time. Seed viability values remained high in seeds stored in the coldroom and warmroom but dropped to almost zero in the warehouse at 20 mo after storage. The loss in viability of untreated seed was significantly greater than that of treated seeds at 16 months in the warehouse while in the coldroom and warmroom the effects were visible at 20 mo after storage. Viability of seeds in the coldroom and warmroom remained higher than 80% at the end of 20 mo of storage. Temperature and relative humidity of the coldroom were best for maintaining seed vigor above 80% for 12 months only. Maturity group and protein content did not affect deterioration. Only 5 to 15% of the decline in seed vigor could be attributed to oil content of the seeds, depending on the storage condition. Treated soybean seeds could be carried-over for two seasons if the temperature of storage is maintained at 10^°oC and the relative humidity kept constant at &le 40%. Seed treatment would improve storability if seeds are stored in low temperature and relative humidity conditions. Prolonging soybean seed viability and vigor of treated seed in storage could reduce the need for disposal of treated seeds. These results are also important for the crop-protection chemical companies because of the importance of seed treatments to the overall crop-protection strategies.