This report is divided into two parts. The first part is a retrospective analysis of 40 unrelated transplants [UD-BMT] for severe acquired aplastic anemia [SAA], performed at five transplant centers between 1981 and 1990. The second is an interim analysis of 15 UD-BMT for SAA compared with a matched cohort of 16 sibling transplants [ID-BMT] carried out since February 1989 using prospectively collected data provided by the IMUST Study Group. In the retrospective study the actuarial survival was 28% [+/- 15] with a median follow-up of 1485 [266-3890] days post BMT. Univariate analysis showed a considerable survival advantage for patients younger than 15 years at BMT [p = 0.03]. More recently patients entered in the IMUST Study showed an actuarial survival of 50% [+/- 27] for UD-BMT compared with 58% [+/- 31] for ID-BMT recipients. These data suggest that UD-BMT is a realistic option for paediatric patients with SAA. Although numbers are very small there is some indication from IMUST Study data that better patient selection and intensive BMT protocols may also improve the results of UD-BMT in young adult patients with SAA.