The serum fatty acid composition reflects the dietary fatty acid composition from the past few days to several weeks. However, the role of serum omega-3 (from fish and fish oils) and omega-6 (from vegetable oils) polyunsaturated fatty acids (PUFAs) in the course of metabolic syndrome is poorly understood. At the Primary Health Care Unit in Pieksämäki, Finland, all subjects born in 1942, 1947, 1952, 1957, and 1962 (n = 1,294) were invited for health checkups in 1997-1998 and 2003-2004. Metabolic syndrome was defined by using the new, harmonized criteria. The serum omega-3 PUFAs, omega-6 PUFAs, and total fatty acids were analyzed by proton nuclear magnetic resonance spectroscopy. Altogether, data from both checkups were available for 665 subjects. After adjustment for age, sex, and baseline body mass index, the incidence of metabolic syndrome between the 2 checkups with a 6.4-year follow-up was inversely associated (P