One hundred and forty-eight patients with tension headaches at least 10 days per month were studied prospectively with a standardized interview, neurological examination and evaluation of muscle tenderness and jaw movements. Patients with migraine attacks oftener than once per month were excluded. Ninety-three had daily headache. Median age at onset was 23 yrs. (quartiles 15–23 yrs.). Half of the patients had headache predominating in one part of the head. In the other half, headache was diffuse or with changing pattern. Fifty-eight reported migraine attacks 1–12 times per year; twenty-seven of these did not remember onset, 15 initially had migraine later developing into chronic headache, and 16 started with tension headache and developed migraine attacks later in life. Photophobia was reported as frequently associated with tension headache (migraine attacks not included) in 13 percent, infrequently in 38 percent and absent in 49 percent. Corresponding figures for phonophobia were 7, 35 and 57 percent. Characteristics of chronic tension headache were similar in patients with and without migraine attacks. The components included in the usual indices of temporo-mandibular joint dysfunction showed little mutual correlation. Pericranial tenderness scores were elevated compared to previously studied controls. Tenderness in paravertebral muscles was significantly correlated to pericranial tenderness, and patients indicating influence by environmental factors had the highest pericranial tenderness scores. The findings are suggestive of a generalized muscular reaction to environmental stress in the majority of patients.