Adult T-cell lymphoma/leukemia (ATLL) is a rare tumour of T-lymphocytes that is associated with human T-lymphotrophic virus type 1 (HTLV-1) infection as well as severe/refractory hypercalcaemia. Human immunodefficiency virus type 1 (HIV-1) infected individuals are at increased risk of acquiring co-infection with HTLV-1. We present the case of a 37 -year -old HIV-1 positive and antiretroviral therapy naive woman who was admitted to the ICU with delirium, a generalised maculopapular rash, severe hypercalcaemia of 4.48 mmol/L (normal < 2.7 mmol/L) and a positive HTLV-1 serology. The diagnosis of ATLL was confirmed on biopsy. Her hypercalcaemic state proved refractory to conventional therapy, but was rapidly corrected with a modified haemodialysis technique using a dialysate with a low low-calcium concentration.