The activation marker CD3O is useful in the diagnosis of Hodgkin and non-Hodgkin lymphomas. It has also been described in nonhematopoietic tumors, including pancreatic carcinomas, salivary gland tumors, and embryonal carcinomas. We report a case of malignant mesothelioma with intense CD30 positivity. This finding has not previously been described and is important in broadening the differential diagnosis of a CD30(+) cohesive large cell malignancy.