Informal care is a substantial source of support for people with cancer. However, various studies have predicted its disappearance in the near future. The aim of this study is to analyse the catastrophic effect resulting from the substitution of informal care with formal care in patients with blood cancer throughout the different stages of treatment. A total of 139 haematological neoplasm patients who underwent stem cell transplantation in Spain, completed a longitudinal questionnaire according to the three phases of treatment between 2012 and 2013. The economic value of informal care was estimated using proxy good, opportunity cost, and contingent valuation methods. Catastrophic health expenditure measures with thresholds ranging from 5 to 100% were used to value the financial burden derived from substitution. A total of 88.5% of patients reported having received informal care. In 85.37%, 80.49%, and 33.33% of households, more than 40% of their monthly income would have to be devoted to the replacement with formal care, with monthly amounts of €2105.22, €1790.86, and €1221.94 added to the 40% in the short, medium, and long-term, respectively (proxy good method, value = 9 €/h). Informal caregivers are a structural support for patients with blood cancer, assuming significant care time and societal costs. The substitution of informal care with formal care would be financially unaffordable by the families of people with blood cancer.