This study describes a new technique for the visualisation and quantitation of glandular epithelial cell proliferation in gastrointestinal mucosa using computerised three-dimensional reconstruction. The tissue used in this study was colorectal biopsy tissue infiltrated in vitro with bromodeoxyuridine (BrdU), although the method could be applied to any gastrointestinal site labelled with any specific marker for cell proliferation. The method is as follows. Five-micron-thick serial sections (> 100) were cut from colorectal biopsies infiltrated in vitro with BrdU. After labelling all the sections for BrdU-positive cells using standard immunohistochemistry, colorectal glands were identified which were completely sectioned within the series. Each microscopic image of the sectioned gland was orientated, digitised and stored using a Kontron image analyser. On each of the stored images, the crypt profile, the positive cells and the negative cells were interactively marked and digitally stored. Using three-dimensional (3-D) reconstruction software, the outer surface of the crypt, the total positive and the total negative fractions could be viewed in three dimensions. The total BrdU-positive cell number could be automatically calculated for the complete crypt or, alternatively, compartmental analysis of the labelling pattern within the crypt could be obtained. This represents a powerful technique: it does not require orientation, it can be carried out on complex glandular structures and is not affected by the biases involved in measuring labelling indices from single tissue sections.