Loneliness is a psychological experience related to social isolation and perceived lack of companionship, and may be relevant to health risk. The revised UCLA loneliness scale was completed by 240 working men and women aged 47-59 years, and related to affective state and neuroendocrine, cardiovascular, and inflammatory responses. Loneliness scores were not associated with gender, age or socioeconomic position, but were lower in married than single or divorced participants, and were positively related to social isolation, low emotional support, ratings of depression, hopelessness and low self-esteem, and to reported sleep problems. Diastolic blood pressure reactions to acute mental stress were positively correlated with loneliness in women but not men, independently of age, socioeconomic status, smoking, body mass and marital status (p = 0.014). Lonely individuals also displayed significantly greater fibrinogen (p = 0.038) and natural killer cell responses (p = 0.042) to stress, independently of covariates. The cortisol response over the first 30 min following waking was positively associated with loneliness after adjusting for waking cortisol value, sex, socioeconomic status, smoking, time of waking, and body mass (p = 0.046). We conclude that loneliness is a psychological experience with potentially adverse effects on biological stress processes that may be relevant to health.