Bone marrow transplant-associated thrombotic microangiopathy (BMT-TM) has been reported with widely varying frequencies and outcomes. Therefore a BMT-TM grading system (0-4) was developed based on the lactate dehydrogenase (LD) level and percentage (%) fragmented cells (FC) as follows: grade 0, normal or increases LD and FC < or = 1.2%; grade 1, normal LD and FC > or = 1.3%; grade 2, increases LD and FC = 1.3-4.8%; grade 3 increases LD and FC = 4.9-9.6%; and grade 4, increases LD and FC > or = 9.7%. Patients with grade 0 and grade 1 BMT-TM did not differ in clinical parameters. Twenty two patients had BMT-TM grade 2-4. These were observed for outcome. Seven of 10 with grade 2 BMT-TM had resolution of BMT-TM a median of 99 days (range 50-229 days) from diagnosis. This occurred spontaneously in five and following discontinuance of cyclosporine (CsA) in two. The remaining three had persistent BMT-TM at grade 2 (two patients) and grade 1 (one patient). In contrast, none with grade 3-4 BMT-TM had resolution. Seven with grades 3-4 BMT-TM underwent a variety of plasma exchange procedures; six had partial hematologic responses. Patients with grades 3-4 BMT-TM had a poorer survival (median survival = 60 days) than those with grade 2 BMT-TM where the median survival has not been reached (P = 0.018). These results indicate that BMT-TM is common following allogeneic-BMT and the outcome is dependent on the grade. Those with grade 1-2 BMT-TM generally do well.(ABSTRACT TRUNCATED AT 250 WORDS)