A somewhat obscure etiologic theory for facial neuralgias presumes a low-grade osteomyelitis of the jaws that produces neural degeneration with subsequent production of inappropriate pain signals. Animal investigations and treatment successes with human patients based on this theory lend it credence. The present study examined 224 tissue samples removed from alveolar bone cavities in 135 patients with trigeminal neuralgia or atypical facial neuralgia. All tissue samples demonstrated clear evidence of chronic intraosseous inflammation. The most common microscopic features included dense marrow fibrosis or "scar" formation, a sprinkling of lymphocytes in a relative absence of other inflammatory cells (especially histiocytes), and smudged, nonresorbing necrotic bone flakes. Very little healing or new bone formation was visible. These lesions were able to burrow several centimeters to initiate distant cavities. The present preliminary investigation cannot prove etiology, but the presence of intraosseous inflammation in every single jawbone specimen in these patients and certain clinical and treatment aspects of these lesions (to be reported later) has led the authors to recommend the term neuralgia-inducing cavitational osteonecrosis or NICO for these lesions.