In 72 of 81 consecutively diagnosed patients with Hodgkin's disease pretreatment sera were available for analysis of serum thymidine kinase (S-TK) levels. In relation to clinical parameters, such as stage, histopathology and general symptoms, significant correlations were found with higher (S-TK) levels in advanced disease as well as in patients with B-symptoms. When the prognostic ability was examined, patients in stages IA and IIA could be divided according to S-TK levels into two different groups in relation to disease-free survival. This latter finding makes this serum test interesting as an additional tool in the clinical evaluation and in the therapeutic decision concerning patients with Hodgkin's disease.