Several experimental models have demonstrated that the central nervous system is functionally linked to the immune system by means of the autonomic nervous system. Samples of 36 lymph nodes of patients whose ages ranged from 16 to 69 y were studied. In order to demonstrate the existence and distribution of sympathetic nerve fibres, a polyclonal antibody antityrosine hydroxylase (TH), with the streptavidin-biotin system of detection, was used. TH-positive nerve fibres appeared in all reactive patterns of the lymph nodes studied. Thin nerve fascicles ramified at the hilar region and also in the connective tissue septae. Adventitial adrenergic nerve fibres were found following afferent, and to a lesser extent, efferent blood vessels. Another source of incoming nerve fibres was found at capsular level, accompanying blood vessels. On the arterial side, the innervation ceased before reaching the follicular arterioles. Our demonstration of innervation in postcapillary venules could support a regulatory role of adrenergic neurotransmitters in lymphocyte traffic. Occasional nerve fibres were also seen in T areas among parenchymatous cells. These findings confirm the existence of sympathetic innervation in human lymph nodes, and provide indirect evidence that the psychoneuroimmune axis could also exist in humans.