In experiments with suspensions of cells from colonic carcinomas, we noted that some colonic carcinomas contain large numbers of eosinophils. We therefore carried out a prospective study with 67 almost consecutive colonic carcinomas in our medical center after optimal fixation and staining for the demonstration of eosinophils. Infiltration of the primary tumor by eosinophils was found to have marked prognostic significance. The proportion (4 of 17 or 23.5%) of carcinomas with more than 30 eosinophils/sq mm that had metastases was significantly less (p = 0.01) than the proportion (31 of 50 or 62.0%) of carcinomas with less than 30 eosinophils/sq mm that had metastases. At 18 months, following the resection of tumor in patients without metastases, all of the patients (9 of 9) with greater than 30 eosinophils/sq mm were alive in contrast to 73.7% (11 of 15) of the patients with less than 30 eosinophils/sq mm. The number of survivors at 18 months for the total population without regard to metastases was significantly greater (p = 0.028) for those with greater than 30 eosinophils/sq mm than for those with less than 30 eosinophils/sq mm. We conclude that the quantitative assessment of eosinophils is one of the most important aspects of the microscopic evaluation of this common human tumor.