Calcineurin inhibitors (CNI) are the mainstay of immunosuppressant medications in both bone marrow transplants and solid organ transplants. Nephrotoxicity is a well-known adverse effect of this group. Type IV renal tubular acidosis is a potentially under-recognized complication. Here we report a case of Omenn syndrome in a patient who underwent a bone marrow transplant and developed type IV renal tubular acidosis while on treatment with cyclosporine.