AIM: To compare the Ki-67 labelling index determined using the enhanced peroxidase one step (EPOS) method with that using the standard ABC technique in pituitary tumours. METHODS: Adjacent sections were immunostained using the EPOS and ABC techniques with the same Ki-67 antibody and same antigen retrieval method. RESULTS: The labelling index measurements with the EPOS Ki-67 antibody were significantly higher than when using the traditional ABC method, and there was a positive correlation between the two techniques when performed on the same cases. This suggests that a higher proportion of cells are within the cell cycle than previously thought, although it gives no direct information on the rate of proliferation of the tumour. CONCLUSIONS: It appears that the EPOS system is not only more convenient but may be more sensitive than traditional techniques for detecting Ki-67 in the nuclei of cells, thus demonstrating more accurately which cells have entered the cell cycle.