Technical Abstract: This study was undertaken to determine if changes in lipid phase behavior could be used to detect lost viability in lettuce (Lactuca sativa) seeds. We used seeds from the cultivar ‘Black Seeded Simpson’ that were purchased every 2-3 years since 1989 and stored in resealable plastic bags at constant 5°C and relative humidity ranging from 30 to 60%. Viability of seeds from each harvest year was recently tested by germination assays carried out on two replicates of 50 seeds each, and seed lots were scored for % and rate of germination, physiological necrosis and abnormal development. Seed lipids were extracted from an aliquot of seed from each harvest year and total lipid content and fatty acid composition were measured. The temperature and energy associated with lipid melting were measured using differential scanning calorimetry (DSC) on whole seed and extracted lipid samples from each harvest year. Reduction of normal germination was evident in seeds after 4 years of storage and % germination was less than 20% after 9 years. However, 100% of seeds germinated (radicle emergence) until 13 years of storage and then it dropped precipitously to 0% by year 18. Amount of extractable lipid appeared to decline in seeds with increasing time in storage. In addition, a significant decline in linoleic acid was noticed after 9 years of storage. The energy of the lipid melting transition of intact seed also declined with time in storage and is significantly correlated with reductions in normal germination. The DSC measurements required no special handling protocols and did not affect seed viability or vigor. Hence, it may be a useful, nondestructive tool for determining the progress of seed aging and help schedule actual germination assays for monitor testing.