The prognostic relevance of phytohemagglutinin (PHA) and purified protein derivative (PPD) lymphocyte reactivity at diagnosis and after radiotherapy was evaluated in breast cancer patients. The patients were followed up to 13 years and the prognostic value expressed as ratios between observed number of deaths and "estimated" number of deaths under the null hypothesis. There was no significant association between the initial PHA and PPD reactivity and the survival of the patients. On the other hand, mortality up to 8 years after radiotherapy was significantly higher for patients with low PHA and PPD reactivity at completion of treatment. Furthermore, patients who had higher than average PPD reactivity 6 to 10 months after radiotherapy, seemed to have a higher survival rate. The prognostic relevance of postirradiation lymphocyte reactivity was only to some extent explained by clinical stage.