The clinical and pathologic features of 15 new cases of the uncommon primary or granulomatous angiitis of the central nervous system (PACNS) are described. To date, only 108 such cases have been reported in the English literature. Clinically, most PACNS patients have been young or middle-aged (mean age, 45 years; range, 3 to 96 years), with men outnumbering women slightly by a ratio of 4 to 3. The most frequent presenting complaints are headache, weakness, and confusion; less common complaints are aphasia, dysphasia, nausea or vomiting, loss of memory, and seizure disorder. There is usually no evidence of a systemic disease; the erythrocyte sedimentation rate is almost invariably normal, and there are no diagnostic laboratory tests. The cerebral angiogram usually shows multifocal, segmental stenosis or irregularity of small and medium-sized leptomeningeal and intracranial blood vessels, often with a beading or aneurysmal appearance, and alterations in blood flow in the affected regions. Anatomically, the angiitis is focal and segmental in distribution. An isolated negative biopsy, therefore, does not rule out the disease. Histologically, PACNS may be granulomatous, necrotizing, or lymphocytic in character, and mixed morphologic types often occur. Large- and small-vessel thrombosis is common. Acute lesions frequently coexist with healing or healed lesions. Involvement of extracranial blood vessels occurs only rarely. Past or current herpes zoster infection and Hodgkin's lymphoma are the most noteworthy clinical associations of PACNS, but whether they are causally related remains uncertain.