Eighty histologically verified cases of the desmoid tumor (DT) have been analyzed with regard to factors possibly contributing to the etiology and/or growth behavior of this uncommon neoplasm. Considering the four statistical age components, the "fertile" female and "menopausal" varieties of the DT grew distinctly faster (0.1 less than P greater than 0.05 and less than 0.05), and the female "juvenile" variety distinctly slower (0.1 less than P greater than 0.05) than the male DTs. In a visual estimate, the fertile female patients had a significant (P less than 0.01) predisposition to estrogen predominance, while fewer patients than expected displayed progesterone predominance or were at balance (P = NS and less than 0.01, respectively). Thirty-two per cent of the patients with an abdominal DT had been previously operated in the region of subsequent tumor growth. Significantly more pregnancies were observed in patients with abdominal DT than with extra-abdominal DT (P less than 0.05). On only one occasion did sigmoideoscopy reveal colonic polyposis (Gardner's syndrome). The most striking observation was, however, that up to 80% of the affected patients (compared with less than 5% in the normal control population, P less than 0.05) had multiple minor bone anomalies demonstrable by x-ray screening of the mandible, chest, and long bones. We suggest that a generalized (inherited or mutant) defect in growth regulation of connective tissue is the most important underlying cause for the DT. However, the other factors, including hormonal effects, trauma, and pregnancy contribute to the growth behavior of the tumor.