We investigated whether immune deficiency was associated with a fatal case of invasive cerebral aspergillosis due to Aspergillus fumigatus infection. The lymphocyte proliferative capacity to T- and B-cell mitogens was comparable to that of healthy controls. However, the natural killer (NK) cell activity of the patient was 2–6-fold lower than the activity expressed by a paired control and age-sex matched healthy controls (n = 20). No specific abnormalities were evident in the capacity to transduce either proliferative signals or cytokine (interleukin 2) secretory capacity of in vitro activated lymphocytes from the patient. The specific reduction in NK cell activity might either indicate that she was a low NK responder, or that the low NK cell activity was primarily or secondarily associated with the fungal infection caused by Aspergillus fumigatus in humans.