Lymphocyte adenosine deaminase (L-ADA) activity, a measure of lymphocyte activity, was estimated in 10 healthy controls and 30 patients with typhoid fever (20 uncomplicated and 10 complicated) at the time of admission, at onset of complications and weekly until recovery. Mean L-ADA activity in healthy controls was 20.49 +/- 3.62 mU/10(6) cells. In uncomplicated patients L-ADA activity was 36.33 +/- 5.09 mU/10(6) cells at time of admission, which is significantly raised as compared to controls. It remained high at the height of the fever and at defervescence. In complicated patients L-ADA activity was significantly low at admission (15.33 +/- 2.35 mU/10(6) cells) and fell further with development of complications (7.86 +/- 4.07 mU/10(6) cells). At defervescence L-ADA activity increased significantly above the control activity (31.24 +/- 5.37). Serial L-ADA activity can be of prognostic significance. A cut-off value of 24 mU/10(6) cells is suggested to predict prognosis and severity of disease. Activity below this indicates a probability of a severe, prolonged course and may help in instituting early and energetic treatment.