To investigate the effect of storage on serum fatty acids, we examined the composition of serum fatty acids in samples from healthy men stored at different temperatures for various periods of time. The following experimental storage conditions were studied: 1) 4 degrees C for three weeks, 2) -20 degrees C for 12 months and 3) -80 degrees C for 24 months. The fatty acid composition did not change after two-week storage at 4 degrees C. At -20 degrees C, there was a decline in the content of docosapentaenoic acid (22: 5, omega 3) after one month, and docosahexaenoic acid (22: 6, omega 3) after three months. However, omega 3-polyunsaturated fatty acid as a whole did not show a significant change. At -80 degrees C, there was a decline of docosapentaenoic acid after three months, arachidonic acid (20: 4, omega 6) after 12 months, and linoleic acid (18: 2, omega 6) after 24 months. As a whole, omega 3-polyunsaturated fatty acids did not change. Omega 6-Polyunsaturated fatty acids significantly declined 5% after 24 months. Therefore, it is recommended that serum fatty acids be measured within two weeks at 4 degrees C, within a few months at -20 degrees C and within one year at -80 degrees C to estimate the composition of the major fatty acids.