In an attempt to find whether or not p53 immunoreactivity in the thyroid gland is restricted to undifferentiated carcinomas and to evaluate the putative prognostic usefulness of its detection, we investigated p53 immunoreactivity in a series of 14 benign thyroid lesions and 65 thyroid carcinomas (12 papillary; six minimally invasive follicular; four widely invasive follicular; 31 poorly differentiated and 12 undifferentiated tumours). Unequivocal nuclear immunostaining for p53 was observed in two widely invasive follicular carcinoma (20.0%), five poorly differentiated carcinomas (16.1%) and in 10 undifferentiated carcinomas (83.3%). The percentage of immunoreactive cells was much smaller in the former groups than in undifferentiated carcinomas. Despite a trend to a more aggressive behaviour of the p53 immunoreactive cases no significant differences in the outcome of patients with positive and negative tumours was found when the comparison was made within each category of carcinomas. We conclude that p53 immunoreactivity can be detected both in undifferentiated carcinomas and in some differentiated and poorly differentiated thyroid carcinomas. Larger series of cases are necessary to evaluate the prognostic usefulness of this finding.