Positive affective states are associated with favorable health outcomes, but the underlying mechanisms are poorly understood. The authors assessed associations between positive affect, cortisol sampled over the day, and inflammatory markers (C-reactive protein and interleukin-6) among 2,873 healthy members of the Whitehall II study. Data for this study were collected in 2002-2004 in London, United Kingdom. Saliva free cortisol was assessed on waking, 30 minutes later, and four times over the day and evening. Positive affect was indexed by aggregating ecological momentary assessments of positive mood over the day. Salivary cortisol averaged over the day was inversely associated with positive affect after controlling for age, gender, income, ethnicity, body mass index, waist/hip ratio, smoking, paid employment, time of waking in the morning, and depression (p = 0.003). There was no association with cortisol responses to waking. The adjusted odds of C-reactive protein >/=3.00 mg/liter was 1.89 (95% confidence interval: 1.08, 3.31) in low- compared with high-positive-affect women, and plasma interleukin-6 was also inversely related to positive affect in women (p = 0.016). Neither inflammatory marker was related to positive affect in men. These results confirm findings from smaller studies relating cortisol with positive affect while suggesting that in women, positive affect is associated with reduced levels of inflammatory markers.