Platelet monoamine oxidase activity (MAO) from 33 cluster headache patients (17 males, 16 females) and 34 migraine patients (16 males, 18 females) was assayed. The kinetic constants (apparent Vmax and apparent Km) and the thermolability, measured as the ratio of the platelet MAO activity after and before heat treatment (+52 degrees C, 30 min), were determined. The MAO activity and Vmax values were significantly lower in cluster headache than in migraine and in both headache disorders compared to a control group (62 males, 66 females). When comparing all groups, Km was not significantly different except for migraine females, who had lower Km values compared to control females. Thermolability was significantly higher in cluster headache than in migraine and in both headache disorders compared to the control group. Smokers of five cigarettes or more per day had significantly lower Vmax values but similar Km and thermolability values compared to those smoking less or nothing. The findings of low maximal velocities and high thermolability of platelet MAO in cluster headache and migraine are suggested to represent constitutionally different enzyme properties.