The cyclin-dependent kinase inhibitor p27Kip1 is a negative cell cycle regulator linking extracellular growth-regulatory signals to the cell cycle machinery in G1. We investigated the pattern and prognostic value of p27Kip1 expression in a population-based group of 203 non-Hodgkin's lymphoma (NHL) patients. The expression of p27Kip1 was identified by immunohistochemistry and correlated with Ki-67 expression and clinical features. Correlation with outcome was determined using uni- and multivariate analysis stratified by clinical grade. Except for very aggressive NHL, there was a negative correlation between p27Kip1 and Ki-67 expression. Low expression of p27Kip1, defined as nuclear p27Kip1 expression in <40% of malignant cells, was predictive of poor survival in indolent and aggressive NHL. However, even in this regard, very aggressive lymphomas behaved differently as those with low p27Kip1 expression tended to do better. Likewise, a high proliferation rate (Ki-67 >40%) was associated with poor survival in indolent and aggressive lymphomas. Multivariate analysis using the proportional hazards model showed that only p27Kip1, and not Ki-67, maintained independent prognostic significance in indolent and aggressive lymphomas (relative risk = 2. 0; P = 0.0095). The low cost and simplicity of this standard immunohistochemistry analysis makes p27Kip1 a promising and suitable prognostic marker in routine diagnostic laboratories in a standard diagnostic panel.