The relationship between vitamin B12 levels and survival was studied in a group of 161 terminally ill cancer patients who were recruited consecutively between 1988 and 1989. Their average age was 74.7 years. The length of survival decreased with the increase in serum vitamin B12 levels (P = 0.0015, Cox model). In multivariate analyses, C-reactive protein (CRP) was the most important prognostic factor in this population, and vitamin B12 provided information independent of CRP in predicting survival. These data indicate that an elevated serum vitamin B12 level is a predictive factor for mortality in patients with cancer, independent of CRP or other factors. Multiplying it by the CRP makes it possible to create a new, easy-to-use prognostic index, which can distinguish different levels of mortality risk at three months.