The kinetics of serum Aspergillus galactomannan, as determined by enzyme-linked immunosorbent assay, was examined in 37 allogeneic stem cell transplant (SCT) recipients treated for invasive aspergillosis (IA). Fifty-eight periods of response ("response episodes") were evaluated. There were 42 response episodes that were considered "treatment failures" and 16 that were considered "good" (that is, complete or partial) responses. At baseline (the first day of each new response episode), the patients who experienced treatment failure and those who had good responses did not differ significantly with regard to median galactomannan index (GMI) value. Thereafter, GMI values significantly increased in the treatment failure group, whereas no significant changes were observed in the good response group (P=.002). An increase in the GMI value of 1.0 over the baseline value during the first week of observation was predictive of treatment failure with a sensitivity of 44%, a specificity of 87%, and a positive predictive value of 94%. We conclude that serial determination of serum GMI values is a useful tool for assessing prognosis of IA in allogeneic SCT recipients during treatment.