A series of 60 cases of oxyphilic (Hurthle cell) carcinomas (HCC) of the thyroid were reviewed to determine whether it is possible to correlate morphologic and clinical features as a means of assessing prognosis. Twenty cases showing predominant solid or trabecular patterns (as described in poorly differentiated carcinomas with a follicular pattern) were selected and the clinicopathological features were investigated. Based on cell size, two groups of solid or trabecular HCCs were identified: The first group (17 cases) was made up of typical large granular oxyphilic cells, and the second (three cases) had small oxyphilic cells. All tumors were reactive for thyroglobulin and for a mitochondrial antigen, selectively marking oxyphilic, mitochondrial-rich cells. Nuclear pleomorphism in individual cells was a common feature, but foci of anaplastic carcinoma were never found. Four cases overexpressed p53 protein and 10 expressed bcl-2 gene product. At follow-up, among the high-stage (pT3-pT4) tumors, seven patients had recurrences or metastases, six of whom were alive with disease or died of disease. In the control group of HCC with predominant follicular patterns, only one of 40 cases had a fatal outcome. The difference was statistically significant. Small-cell patterns and a p53 protein-positive/bcl-2 gene product negative phenotype were features of clinically aggressive HCC cases. We suggest that within the spectrum of oxyphilic (Hurthle cell) tumors, poorly differentiated HCC showing solid or trabecular patterns are a distinct group, based on both morphological and clinical features.