Clinical and immunological data are reported of 12 patients suffering from Wegener's granulomatosis and severe renal involvement. Although 9 patients recovered from their acute illness, a long-term follow-up a relapse occurred in 4 of these 9 patients. Therefore, lifelong follow-up in this group patients seems to be mandatory. Extensive immunological investigations did not provide evidence for humoral mechanisms underlying the pathogenesis of this disease; T lymphocyte subsets in peripheral blood as well as functional reactivity of lymphocytes in vitro were also normal. However, none of the patients was able to mount a primary cellular immune response in vivo. On the other hand, kidney biopsy specimens obtained before the initiation of drug therapy revealed periglomerular and interstitial cellular infiltrations consisting predominantly of T lymphocytes with a ratio Leu 3a (OKT4)/Leu 2a (OKT8) of 5:1. This may indicate that a type IV (delayed-type) hypersensitivity reaction takes place in the kidney. These findings suggest that an abnormal cellular immunoreactivity plays a major role in the pathogenesis of Wegener's granulomatosis.