This is a phase I clinical trial to evaluate the safety, tolerability, and maximum tolerated dose of DMF in patients with chronic lymphocytic leukemia. Patients with relapsed/refractory CLL not amenable to available therapies are eligible. This patient population is in need of novel therapies, particularly if progressing after, intolerant of, or unable to receive oral tyrosine kinase inhibitors (ie ibrutinib, idelalisib).