When human serum must be stored for extended periods before analysis, conditions must be such that samples give results comparable with those for "fresh" (unstored) samples. In a balanced experiment with statistically pre-determined sample size, we assessed, at three-weekly intervals for 18 weeks, the stability of total cholesterol, high-density-lipoprotein cholesterol, and triglycerides in human serum stored at -15 degrees C. No significant changes were observed.