Previous studies have identified high numbers of intraepithelial T lymphocytes to be associated with good prognosis in various types of cancer. Few studies addressing this issue have been published for colorectal cancer. In a simulated prospective approach ("phase II prognostic factor study"), all nonmetachronous International Union Against Cancer (UICC) stage III colorectal cancers that were accessioned in the years 1994 to 1999 were included in the study (152 cases). Follow-up information as to vital status and occurrence of metachronous metastases could be obtained for all patients in the years 2001 and 2002. CD8+ intratumoral lymphocytes were quantified after immunostaining and referred to tumor cell area (CD8+ densities). Microsatellite status was determined by using the Bethesda panel of microsatellite markers. CD8+ densities ranged from 0 per square millimeter to 1436 per square millimeter of tumor area in a nonnormal distribution that was skewed toward low values. Univariate survival analyses revealed the 66th percentile as a stringent cutoff (CD8+high versus CD8+low), with CD8+high cases taking a significantly better clinical course. This prognostic impact appeared even more pronounced in the subset of patients with colon carcinomas who were receiving 5-fluouracil/leucovorin as adjuvant treatment (79 patients). Seventeen patients had carcinomas with high microsatellite instability (MSI-H). MSI-H-CD8+high cases (n = 11) showed an excellent prognosis, with tumor-free survival for 9 of the 11 patients. The prognostic effect of CD8+high was retained in Cox regression analyses when including UICC substages (IIIA to IIIC). Our results identify CD8+ tumor-infiltrating lymphocytes as a promising candidate for further evaluation in the ongoing search for prognostic and predictive factors of colorectal cancer, particularly if combined with microsatellite status.