The role of genetic factors in the pathogenesis of agranulocytosis was investigated in agranulocytosis patients by phenotyping for N-acetyltransferase and glucose-6-phosphate polymorphism; by typing for gene products of the major histocompatability complex, ABOand RH-blood groups, and haemoglobins; and by performing cytogenetic analysis of chromosome aberrations. Nine persons were identified as agranulocytosis cases in the period from 1982 to 1987 among the population of Sofia. They were contacted again 10 years after recovery from the disease. Five of them were associated with metamizol (dipyrone) use. The results obtained revealed significant differences between the agranulocytosis patients and the healthy population in the human lymphocyte antigen (HLA) allele frequencies, and in the degree and the frequency of chromosome aberrations. A higher frequency of the HLA24 antigen (relative risk 13.60,p=0.05) and a lower frequency of the DQAl*0501 allele were evident for the ex-agranulocytosis patients as compared to the controls (11% versus 57% respectively,p=0.05). In the patients exposed to metamizol, an A24-B7 haplotype was found with a frequency higher than that in the non-exposed patients and the reference group (p<0.05). The HLA-DQwl antigen and metamizol-related agranulocytosis were evidently associated in all cases (5/5; 100%) in contrast to the patients not exposed to metamizol and the controls. The HL-A2 antigen was absent in four of the five metamizol-associated agranulocytosis cases (20%), while in the control group it was present in 56%. The degree of structural rearrangements (0.62 ± 0.2%) and the frequency of chromosome breakages (7.75 ± 0.68%) in agranulocytosis patients were higher than those in the healthy population (0.3 ± 0.12%,p<0.05 and 1.42 ± 0.27%,p<0.01, respectively). The abnormalities affected predominantly chromosomes 1(1p13), 2(2pl2) and 5(5pl2). No differences were found between the agranulocytosis patients and the healthy population when considering the haemoglobin subtypes, ABO-and RH-blood groups, glucose-6-phosphate dehydrogenase activity and the rates of slow and rapid acetylators