To study the usefulness of the enzyme adenosine deaminase for the early diagnosis of typhoid fever, its activity in serum was assayed in 277 children admitted to the Hospital Guillermo Grant Benavente at Concepción, Chile, from March, 1988, to December, 1990. The children were distributed into seven groups: control, N = 82; bacteremia, N = 8; acute viral respiratory infection, N = 43; febrile children with miscellaneous etiologies, N = 49; pulmonary tuberculosis, N = 3; hepatitis A virus infection, N = 30; and typhoid fever, N = 62. The medium serum adenosine deaminase values were significantly higher in children with typhoid fever (P < 0.0001) in relation to the values in the control group (122.2 +/- 40.7 vs 28.1 +/- 8.4 units/liter at 37 degrees C). This test had a sensitivity of 91.9% and a specificity of 92.5% in identifying the patient with typhoid fever when using 80 units/liter as the cutoff values. The positive predictive value of the test was 83.8% and the negative predictive value was 96%. Determination of adenosine deaminase values in serum could be helpful in the early diagnosis of typhoid fever.