Data on diagnostic performance of Galactomannan (GM) testing in patients under mould-active regimens are limited. Whether sensitivity of GM testing for diagnosing breakthrough invasive aspergillosis (IA) is decreased under antifungal prophylaxis/therapy remains therefore a point of discussion. We retrospectively analysed GM test results in patients who were admitted with underlying haematological malignancies to two Divisions of the Medical University Hospital of Graz, Austria, between 2009 and 2012. Only cases of probable and proven IA that were diagnosed by other methods than GM testing were included (time of diagnosis = day 0). We compared GM results of patients with/without therapy/prophylaxis for the period of 2 weeks prior (week −2) until 3 weeks postdiagnosis. A total of 76 GM test results in nine patients were identified. Six patients had received antifungal therapy/prophylaxis from week −2, whereas three patients were treated with therapy from the time of diagnosis at week 0. GM testing was positive in 45/76 (59%) of samples. Sensitivity of GM testing for detection of proven or probable IA at week −1 and 0 was 77% and 79% in patients with mould-active regimens. We conclude that GM testing might be a useful diagnostic method for breakthrough IA in patients receiving mould-active prophylaxis/therapy.