We developed a novel algorithm to define the need for high-dose prophylactic i.v. Igs (IVIG) in periods of high risk for CMV to patients after allo-SCT. IVIG were administered only if at least one of the following, monthly-assessed, criteria was fulfilled: (1) IgG concentration <4 g/l, (2) NK (natural killer) cell count <100/microl, (3) CD4(+) cell count <100/microl, (4) acute or chronic GVHD. The primary endpoint was to determine the cumulative incidence of CMV infection in patients who received prophylactic IVIG according to the algorithm (intervention group) and compare it with that of a sequentially assessed control group, to which prophylactic IVIG were not administered. The study included 79 patients (44 in the intervention and 35 in the control group). The estimated cumulative incidence of CMV infection in the intervention and control group did not differ significantly (44 and 36%; P=0.31). Additionally, prophylactic IVIG did not reduce the frequency of CMV infection episodes. CMV disease was rare in both cohorts (5 and 9%; P=0.65). We conclude that prophylactic IVIG should not be administered after allo-SCT, even if administered selectively in a high dose to patients with delayed immune reconstitution or GVHD.