Vascular headaches, including migraine, cluster, and migrainous transformation to chronic daily headaches, are disabling. During and shortly after headache intervals, difficulties are reported in concentration, comprehension, and communication, not accounted for by nausea, photophobia, or sonophobia. These interfere with interpersonal relations and performance at work with economic loss. The hypothesis tested and reported here is that cognitive impairments comprise an important part of vascular headache diatheses. One hundred ninety-six otherwise normative subjects suffering from migraine or cluster, but not tension-type, headaches (136 women, 63 men; mean age, 46 years) participated in an outpatient prospective trial. One hundred thirty-three patients had migraine without aura, 39 migraine with aura, 11 periodic cluster (by IHS criteria), and 13 had migrainous transformation into chronic daily headaches. Neuropsychological testing was compared with and without headaches, by combined Mini-Mental Status Examination (MMSE), Cognitive Capacity Screening Examination (CCSE), and Hamilton Depression Rating Scale (HDRS). During headache intervals, significant decline was measured in both CCSE and MMSE scores (P <. 001) without HDRS change in all types of vascular headache and independent of headache severity, which often improved, or associated physical symptoms. Cognitive decline was promptly relieved by serotonin agonists and sleep. Disorders of cerebral serotoninergic projection systems appear to cause these reversible cognitive impairments.