Negative affective states such as depression are associated with premature mortality and increased risk of coronary heart disease, type 2 diabetes, and disability. It has been suggested that positive affective states are protective, but the pathways through which such effects might be mediated are poorly understood. Here we show that positive affect in middle-aged men and women is associated with reduced neuroendocrine, inflammatory, and cardiovascular activity. Positive affect was assessed by aggregating momentary experience samples of happiness over a working day and was inversely related to cortisol output over the day, independently of age, gender, socioeconomic position, body mass, and smoking. Similar patterns were observed on a leisure day. Happiness was also inversely related to heart rate assessed by using ambulatory monitoring methods over the day. Participants underwent mental stress testing in the laboratory, where plasma fibrinogen stress responses were smaller in happier individuals. These effects were independent of psychological distress, supporting the notion that positive well-being is directly related to health-relevant biological processes.