Paraproteinaemia following allo-SCT is common. We analysed 91 consecutive patients undergoing allo-SCT; conditioning included alemtuzumab in 42% of the patients. Paraproteinaemia incidence at 2 years was 32%. In univariate analysis paraproteinaemia was associated with unrelated donor, age, recipient seropositivity for CMV and alemtuzumab conditioning (hazard ratio (HR) 3.93, P=0.0006). Paraproteinaemia was not associated with haematological diagnosis; disease status at transplant; varicella zoster, herpes simplex or EBV serology; reduced-intensity vs myeloablative conditioning or GVHD. CMV reactivation-more frequent in alemtuzumab recipients-was associated with paraproteinaemia (HR 7.52, P<0.0001). In multivariate analysis, only increasing age (HR 1.04 per year, P=0.048) and CMV reactivation (HR 5.74, P=0.001) were significantly associated with paraproteinaemia. Alemtuzumab without CMV reactivation, however, resulted in significantly more paraproteinaemia, suggesting an effect that is independent of CMV reactivation. OS was poorer in patients with paraproteinaemia (HR 2.54, P=0.04) and relapse increased (HR 2.38, P=0.087). Paraproteinaemia was not significantly independently associated with decreased survival on multivariate analysis. Post transplant paraproteinaemia is associated with CMV reactivation, is more frequent in alemtuzumab-conditioned transplants and is not associated with improved OS.