Background. There is a practical need to investigate the performance of the serum galactomannan (GM) assay in hematology patients with a potentially low pretest risk of invasive aspergillosis following effective antimold prophylaxis. Methods. We present a 4-year study with 262 unselected consecutive high-risk episodes, prospectively managed with posaconazole primary prophylaxis and a uniform diagnostic algorithm, including biweekly serum GM quantification for early detection of invasive aspergillosis. Results. A total of 2972 serum GM tests were performed (median, 11 per episode range, 3–30); the vast majority were negative (96.7% of tests and 83.6% of episodes). The incidence of breakthrough invasive aspergillosis was 1.9% (5/262), all with true-positive GM test results. Our study identified 30 false-positive GM evaluable episodes (85.7%; 13.8% of all evaluable episodes), validating with real-life data the low positive predictive value of the assay in this setting (12%). In 26 of these 30 episodes (86.7%), the false-positive result(s) occurred in tests performed as preemptive surveillance only. Conversely, in evaluable cases with positive GM tests and a clinical suspicion of invasive fungal disease, the performance of diagnostic-driven GM tests improved, with a positive predictive value of 89.6%. Conclusions. The low pretest risk of invasive aspergillosis in the context of effective antimold prophylaxis renders serum GM surveillance of asymptomatic patients unreliable, as all results would be either negative or false positive. The test remains useful to diagnose patients with a clinical suspicion of invasive fungal disease, calling for a more efficient copositioning of effective prophylaxis and GM testing in this clinical setting.