Computerised morphometry was used to define the distribution of aortic atheroma in 10 normal White Carneau pigeons (age 42 months) and in groups of 10 weanling or mature birds given a cholesterol coated diet for 24 weeks. Mean serum total and HDL-cholesterol was substantially higher in cholesterol fed animals (15 v 8 mmol/l). The spatial distribution of disease was identical in each group. Lesions developed just proximal to the first major abdominal branch and were more plentiful on the right hand side. The major histological component of the lesions was fibrous intimal thickening and when studied separately its distribution closely matched that of the overall lesion. Foam cell aggregates were more plentiful on the right. This distribution pattern is very similar to that seen in early human atheroma and supports the hypothesis that lesions form in regions of low flow velocities.