There is currently no standardized approach for the treatment of steroid-refractory acute GvHD (SR-aGvHD). Alemtuzumab has been shown to have clinical activity in a number of small prospective and retrospective studies, but optimal doses remain unknown and survival remains poor, with deaths from both GvHD and infection contributing. Critically, very little is known about alemtuzumab's pharmacokinetics and pharmacodynamics in this context, and therefore dosing is empirical, ranging from 10–50 mg in the first week, with or without repeat dosing at 10–30 mg every 1–4 weeks. In order to better define the clinical outcome and develop a rational approach to alemtuzumab dosing in SR-aGvHD, we conducted a phase II clinical study in which we incorporated weekly pharmacokinetics and immunological monitoring.