We tested the hypothesis of teVelde and Haak that the degree of bone marrow inflammation in aplastic anaemia might correlate with an immunological process responsive to immunosuppressive therapy. 120 patients with aplastic anaemia but no suitable marrow donor were treated with horse antithymocyte globulin (ATG) and 53 who had matched sibling donors with bone marrow transplants. Pretreatment bone marrow histology in methacrylate and paraffin specimens was graded by degree of inflammatory infiltrate in a four-tiered system. High grade (II-III) was compared to low (O-I) as a correlate of response to ATG. Complete and partial response to ATG was seen in 50% of patients with high grade marrow and 31% of patients with low grade marrow (P = 0.099). Only one of four patients with grade III inflammation responded significantly to ATG treatment. Median survival following ATG therapy was similar in both groups as well. There was a significantly lower median age in the patients with low grade (24.5 years) versus high grade (37.5 years) inflammation (P = 0.016). Grade also had no prognostic value in the marrow transplant group.