Two experiments investigating the aetiology of chronic tension headaches are reported. The first compared neck and forehead EMG of tension headache patients with that of non-headache control subjects whilst at rest and whilst performing a stressful task. The two groups were also compared in terms of pain threshold, and the effect of a vasodilating agent on headache intensity was assessed. In general, EMG levels were similar in the two groups and there were few clearly significant differences. The forehead EMG of the headache patients was marginally higher than that of the control subjects when at rest, but this trend was reversed under stress. In the second experiment, tension headache patients received either EMG feedback training or progressive relaxation training. Results showed the two treatment conditions to be equally effective for reducing recorded headache activity. Headache reduction was not related to success at lowering forehead or neck EMG in the laboratory. Within subject correlations between forehead or neck EMG, and headache intensity at the time of EMG recording, failed to reach significance. Comparing subjects when experiencing headaches and when headache-free revealed no difference in neck EMG, but significantly lower forehead EMG during headaches. The results were interpreted as failing to support the widely-held belief that chronic tension headaches are primarily caused by excessive tension in the skeletal muscles of the head or neck.