To study the changes in immune system during aging and their effects on patient survival, immune functions of 18 very old (92—107 years) hospital patients were tested. After three years follow-up, the survival of patients and the correlation of immune capacity to lifetime were determined. In the very old patients, the frequency of T lymphocytes was significantly lower than in young adults. This decrease was attributed more to the CD8-positive than to CD4-positive cell subset, leading to generally high CD4/CD8 ratios. The mitogenic responses to phytohemagglutinin (PHA) and concanavalin a (Con A), but not pokeweed mitogen (PWM), were decreased significantly in cultures with isolated lymphocytes, while in whole blood cultures the responses to PHA and PWM but not to Con A were reduced. In very advanced age, there was a significant decrease in the number of B cells, and the ability to produce IgM in vitro was decreased. During the follow-up time, 15 out of the 18 patients died. However, their lifetime was not in correlation with any single immune parameter