Chronic lymphocytic leukaemia (CLL) is the most frequent form of leukaemia among adults in the Western world, presenting at a median age of 65 years. The diagnosis is usually made incidentally during routine blood examination while the disease is still in its early phase. We report a case of blindness of 24 hours due to acute sinusitis based on CLL localisation in a patient with undiagnosed CLL. Emergency endoscopic sinus surgery and intra- and extra-ocular orbital decompression were performed. The sinusitis resolved after surgery and intravenous antibiotics. Her vision improved within 24 hours and eventually recovered completely after six months. Her CLL remained in an indolent state, needing no active treatment. This case illustrates that blindness from a lymphoproliferative disorder may be treated with emergency endoscopic sinus surgery instead of conventional chemotherapy in order to salvage the vision first, even if the vision is lost for more than 24 hours.