The unexpected occurrence of thrombotic microangiopathy (TMA), characterised by microangiopathic haemolytic anaemia and thrombocytopenia, in a patient with cancer requires urgent diagnosis and appropriate management. TMA in patients with metastatic cancer can be a manifestation of the malignancy itself or a therapeutic complication. Distinguishing the cause of TMA is complicated but clinically important to initiate appropriate treatment of metastatic cancer and avoid potential drug toxicity. Eculizumab, which inhibits alternative complement pathway activation, has been shown to be effective in chemotherapy-induced TMA. We report the case of a 69-year-old woman with breast cancer who experienced a mitomycin-C-induced TMA manifestation. TMA did not respond to conservative therapy, plasmapheresis or rituximab and rapidly lead to dialysis dependency. Despite disease progression and metastases, eculizumab treatment was associated with recovered renal function and enabled the patient to avoid dialysis, improving her quality of life.