The conflicting results about the prognostic impact of tumour cell proliferation in colorectal cancer might be explained by the heterogeneity observed within these tumours. We have investigated whether a systematic spatial heterogeneity exists between different compartments, and whether the presence of such a systematic heterogeneity has any impact on survival. Fifty-six Dukes' stage B colorectal cancers were carefully morphometrically quantified with respect to the immunohistochemical expression of the proliferative marker Ki-67 at both the luminal border and the invasive margin. The proliferative activity was significantly higher at the luminal border compared with the invasive margin (P<0.001), although the two compartments were also significantly correlated with each other. Tumours with low proliferation at the invasive margin had a significantly poorer prognosis both in univariate (P = 0.014) and in multivariate survival analyses (P = 0.042). We conclude that Dukes' B colorectal cancers exhibit a systematic spatial heterogeneity with respect to proliferation, and tumours with low proliferation at the invasive margin had a poor prognosis. The present data independently confirm recent results from the authors, and provide new insights into the understanding of tumour cell proliferation in colorectal cancer.