Little is known about the causes of health complaints associated with work with video display units (VDUs). The symptoms are to a large degree similar to those of "multiple chemical sensitivity." We observed 47 white-collar employees with and without VDU-associated skin complaints during a regular workday and a day of leisure. VDU workers with skin symptoms had higher levels of the stress-sensitive hormones thyroxin and prolactin compared with employees without symptoms. They also had lower levels of the anabolic hormone testosterone during work. VDU workers with skin complaints also reported more occupational mental strain. A model is proposed in which physiological signals act as unconditioned stimuli and the VDU environment as the conditioned stimuli.