Reference change values of six biochemical quantities (beta 2-microglobulin, neopterin, adenosine deaminase and immunoglobulins IgA, IgG and IgM) have been established in asymptomatic human immunodeficiency virus (HIV)-infected patients following the method described by Harris and Yasaka in 1983. Patients included in the evaluation were classified as A1, A2 or A3 according to the classification of the Centers for Disease Control (CDC) (January 1993). All patients were followed-up quarterly, with a minimum of four samples each available for statistical analysis. The main objective of this paper was to study whether differences found to be greater than calculated reference change values could predict clinical or immunological worsening in patients' status. Retrospective analysis was made in asymptomatic patients (n = 256) included in an HIV infection protocol carried out in our hospital. Of these patients, 179 showed clinical or immunological worsening during the study period and 77 maintained their clinical and immunological status. Changes in beta 2-microglobulin showed the greatest sensitivity to detect clinical or immunological worsening (43.0%), whereas changes in adenosine deaminase showed the lowest (21.8%). Clinical or immunological worsening in 169 of the 179 patients was detected by one of the six biochemical quantities evaluated. Ten patients showed clinical or immunological worsening, although differences between measurements were lower than the reference change values calculated. Of 77 patients whose clinical state did not deteriorate, there was a change in biochemical analytes greater than the reference value calculated in 29 patients (a period of 12 months had elapsed since detection). In 48 patients, no increases greater than calculated reference change values were detected. The sensitivity obtained using the six analytes was 94.4% and the specificity was 62.3%.