Cancer incidence and mortality do not fully reflect the effect of cancer. To estimate the number of persons alive who have a history of cancer, we derived prevalence rates based on data from the Connecticut Tumor Registry. We did not attempt to distinguish between people who had been cured of cancer and those who still had the disease. In 1982 the age-adjusted prevalence rates of cancer among males and females were 1,789 and 2,222, respectively, per 100,000. Age-specific prevalence rates were highest among the elderly; 12 percent of men and 11 percent of women over 70 had previously been given a diagnosis of cancer. Breast cancer in females and prostate cancer in males were the two most prevalent malignant diseases. We estimate that about 5 million persons alive in the United States today have at one time received a diagnosis of cancer.