Forty-five patients with papillary or follicular thyroid carcinomas with tracheal infiltration were examined by clinical, histologic, and morphometric analyses. Twenty-four patients with completely encapsulated carcinomas (encapsulated cases) and 26 patients with carcinomas confining within the thyroid capsule (nonencapsulated cases) were examined as controls. Patients with tracheal infiltration were significantly more often male and older than the patients without (P<0.01 and P<0.05, respectively). The histologic grade of differentiation did not correlate with tracheal infiltration. The nuclear area of tumor cells was significantly larger in the cases with tracheal infiltration than in the cases without (P<0.01). In 12 recurrent cases with tracheal infiltration, the nuclear area of recurrent tumours was significantly larger than those of their own primary tumors (P<0.01). These results confirm that thyroid carcinomas with tracheal infiltration were more frequent in male and older patients and had more significant nuclear atypia than the tumors without tracheal infiltration.