Insulin-dependent diabetes mellitus (IDDM) is associated with an increased risk of coronary artery disease (CAD). There is some evidence that polyunsaturated fatty acids of the marine n-3 type (n-3 PUFA's) may offer protection against CAD. We have studied the effect of short-term dietary supplementation with n-3 PUFAs on lipids, haemostasis, neutrophil and monocyte chemotaxis in 10 patients with IDDM. The patients were given 4 g daily of n-3 PUFAs (fish oil) for 6 weeks and were investigated before and after the supplement. No significant effects on platelets or haemostasis were observed. High density lipoprotein (HDL)-cholesterol significantly increased, and triglycerides and the ratio of total cholesterol to HDL-cholesterol significantly decreased. Monocyte chemotaxis was unaltered, while neutrophil chemotaxis significantly increased after fish oil. The finding of an improvement in neutrophil chemotaxis after supplementation with n-3 PUFAs to patients with IDDM needs to be confirmed in future studies.