In addition to the tumor suppressor gene p53, Cyclin Dependent Kinases (CDK) are well known to influence the cell cycle in normal human tissues and various neoplasias as well. The purpose of our present study was to evaluate the expression of the CDK-inhibitor p21/waf1/cip1 in colorectal cancer with special emphasis on the prognostic impact. Between 1985 and 1991, 294 patients (median age, 65 years) underwent surgical operative therapy for colorectal cancer. Formalin-fixed and paraffin-embedded tumor specimens were investigated. For immunohistochemistry the Catalysed Reporter Deposition (CARD) technique was performed. The survival probability was calculated and possible prognostic risk factors were tested using multivariate analysis. The p21/ waf1/cip1 staining pattern was positive in 197 (67%) specimens and negative in 97 (33%) samples. No significant correlation could been calculated between p21/waf1/cip1 expression and other variables such as age, sex, WHO-Classification, localisation, grading, TNM-classification or UICC-stage. Patients with a positive staining reaction had a significantly better survival (p < 0.0052). Moreover, p21/waf1/cip1 was shown to be an independent prognostic parameter by multivariate analysis (p < 0.022). In contrast with these findings, the p53 tumor status had no impact on survival. P21/ waf1/cip1 appears to be an independent prognostic parameter in colorectal cancer and is associated with a favorable survival. This feature may be related to a cell cycle arrest in the G1 phase induced by p21/waf1/cip1, resulting in lower tumor cell proliferative activity.