Time trends in serum cholesterol were analyzed in the Framingham Study data to determine whether low serum cholesterol, which appears to be associated with increased cancer risk, can truly be called a precursor to the development of the disease. Serum cholesterol measured at base line was found to be associated with the risk of both fatal cancer and all cancers occurring in the subsequent 18 years in men (P less than 0.05). This inverse association seems particularly strong for colon cancer in men. Analysis of time trends in the Framingham data did not yield consistent results for all ages. Men 50--59 years old had a statistically significant lower serum cholesterol measured 16--18 years before cancer diagnosis. For men 60--65 years old, a statistically significant trend toward lower serum cholesterol was apparent as measurement was made closer to the time fo cancer diagnosis. Although the Framingham data are not conclusive, they do suggest that in some cancer cases where the serum cholesterol level was lower than that expected at as much as 16--18 years before cancer diagnosis, the depressed level was likely to be a precursor to the tumor growth. However, consistent with the metabolic consequences of tumor growth, the data show that in some cancer cases, serum cholesterol had decreased at measurements made close to the time of cancer diagnosis.