QX disease causes mass mortalities among Sydney rock oysters (Saccostrea glomerata). To overcome commercial production losses, Industry & Investment NSW has been developing mass selected QX disease-resistant breeding lines since 1997. This breeding programme has significantly reduced QX-associated mortality in the Lime Kiln Bar (LKB) breeding line relative to non-selected, wild-type (WT) oysters. The current study assessed mortality in families produced by single-pair mating between LKB and WT oysters. When these families were grown in a QX disease-prone area, the progeny of LKB × LKB crosses had significantly lower mortality compared with LKB × WT or WT × WT families. Mortality in the different crosses was associated with infection by sporulating Marteilia sydneyi, the parasite responsible for QX disease. Overall, the study identified a strong association between parentage and mortality resulting from QX disease.