The levels of CD98 antigen expression were studied in 62 consecutive cases of adult acute leukemia including 24 acute lymphoblastic leukemia (ALL) and 38 acute myeloid leukemia (AML) using the monoclonal antibody CAF7 and flow cytometry. The mean follow-up was 13.5 months. The mean relative fluorescence intensity (MIF) of CAF7 varied between 6 and 83 channels (256 channels resolution). No correlation was established between CAF7 cell surface density and most of the predictive parameters such as age, sex, blood counts, immunophenotype, proliferative index (PI) or DNA index. Nevertheless expression of CAF7 correlated positively with survival duration (mean 210 vs 391 days, P = 0.048) and complete remission (CR) duration (mean 132 vs 361, days P = 0.032). The levels of CAF7 differed significantly between ALL and AML (P < 0.001), the ALL cases being all CAF7intermediate or CAF7high. In the AML group the low levels of CAF7 expression correlated with shorter CR duration (mean 132 vs 414 days, P = 0.017). The lack of correlation with other clinical and biological parameters suggested that CAF7 might have an independent prognostic significance in adult AML. Although PI was also positively related to survival duration (P = 0.02), it did not correlate with CR duration or the expression of CAF7. We suppose that the prognostic impact of CD98 is related to the control of cell growth and survival in which the molecule normally participates.