An assessment in Nordic immunohistochemical Quality Control (NordiQC) revealed that only 23% participant laboratories performed optimal staining for detection of cyclin D1 (CyD1) in mantle cell lymphoma (MCL). False-negative results were secondary to suboptimal protocols. We compared the 5 anti-CyD1 antibodies (monoclonal SP4, P2D11F11, and DCS-6 and polyclonal CP236 and 06-137) used in the Scandinavian laboratories. Evaluated were 31 MCLs, 16 other malignant lymphomas, and 19 samples of normal tissues. Sensitivity was as follows: CP236, 100%; SP4, 95%; P2D11F11, 90%; DCS-6, 84%; and 06-137, 53%. SP4 produced the strongest staining. Correlation of CyD1 with the proliferative index was best with polyclonal antibodies CP236 and 06-137. The use of heat-induced epitope retrieval in an alkaline buffer such as 1/10 mmol/L of Tris-EDTA buffer, pH 9, seemed mandatory. For the optimal detection of CyD1 expression, both SP4 and CP236 antibodies should be available in the laboratory.