The effect of a daily supplement with 4 g of n-3 polyunsaturated fatty acids (PUFA) for 9 months to 24 healthy volunteers on neutrophil and monocyte chemotaxis was studied using the under-agarose technique. Autologous serum and n-formyl-methionyl-leucyl-phenylalanine were used as chemoattractants. The effect after 9 months of supplementation with n-3 PUFA was also compared to results after short-term supplementation with n-3 PUFA for 6 weeks. Monocyte chemotaxis was reduced after 9 months of supplementation with n-3 PUFA to the same extent as after 6 weeks supplement. Neutrophil-directed migration towards chemoattractants was reduced after 9 months on fish oil, and this decrease was significantly greater than the decrease obtained after 6 weeks of supplementation. The spontaneous migration of neutrophils was significantly attenuated after 9 months compared to baseline and to 6 weeks. These findings lend support to a role for n-3 PUFA in the management of chronic inflammatory and atherosclerotic vascular diseases.