A seven-year-old male Pomeranian with unstable insulin-dependent diabetes mellitus developed atherosclerosis. Hyperlipidaemia, consisting of hypertriglyceridaemia and hypercholesterolaemia, was diagnosed concurrently with the onset of lipid-laden aqueous humour. Serum lipoprotein analysis was characterised by the presence of chylomicrons, an increase in the very low density lipoprotein fraction, with a broad very low density lipoprotein-low density lipoprotein band, and a reduced high density lipoprotein fraction. The dog developed ketoacidosis one year later and died. At postmortem examination, atherosclerotic plaques were observed in the terminal aorta and in medium-sized arteries, including the coronary arteries, renal and arcuate arteries, and arteries of the brain. Mineralised plaques or complicated plaques were not observed. The absence of clinical signs of organ ischaemia was thought to be associated with the absence of thrombosis and/or complete occlusion of all vessels examined. Signs of chronic organ hypoxia, although considered likely owing to the severe vessel lumen reduction, were restricted to only a low voltage QRS complex on the electrocardiogram.