In humans, large granular lymphocytic leukaemia (LGLL) is a low-grade, indolent lymphoproliferative disorder of large granular lymphocytes (LGL) associated with autoimmune disorders; including rheumatoid arthritis and single or multiple cytopenias; particularly neutropenia. Therapy largely centres around immunosuppression which aims to resolve the immune-mediated secondary pathology, often without eradicating the neoplastic clone. The most effective agents appear to be cyclophosphamide, cyclosporine and methotrexate. This case report describes the presentation, diagnostics, therapeutic approach and outcome of a 6 year-old Golden Retriever presenting with severe neutropenia. Chlorambucil, prednisolone and cyclosporine failed to improve the neutropenia but subsequent cyclophosphamide resulted in a sustained albeit temporary improvement in neutrophil count and the ability to withdraw prophylactic antibacterials. This case closely mirrors the diagnostics and therapeutic response in human LGLL.