AbstractSecondary immunodeficiency (SID), manifesting as increased susceptibility to infection, is an emergent clinical problem in haematoncology. Management of SID includes vaccination, prophylactic antibiotics (pAbx) and immunoglobulin replacement therapy (IgRT). We report clinical and laboratory parameters of 75 individuals, treated for haematological malignancy, who were referred for immunological assessment due to recurrent infections. Forty‐five were managed with pAbx while thirty required IgRT after failing to improve on pAbx. Individuals requiring IgRT had significantly more bacterial, viral and fungal infections resulting in hospitalization at least 5 years after their original haemato‐oncological diagnosis. Following immunological assessment and intervention, a 4.39‐fold reduction in the frequency of hospital admissions to treat infection was observed in the IgRT cohort and a 2.30‐fold reduction in the pAbx cohort. Significant reductions in outpatient antibiotic use were also observed in both cohorts following immunology input. Patients requiring IgRT were more hypogammaglobulinaemic and had lower titres of pathogen‐specific antibodies and smaller memory B cell populations than those requiring pAbx. Test vaccination with pneumococcal conjugate vaccine discriminated poorly between the two groups. Patients requiring IgRT could be distinguished by combining wider pathogen‐specific serology with a frequency of hospital admissions for infection. If validated in larger cohorts, this approach may circumvent the need for test vaccination and enhance patient selection for IgRT.