In contrast with various forms of lymphadenitis, the presence of reactive monocytoid B-cells (MBCs) has only rarely been reported in Hodgkin's disease (HD). In order to analyse their occurrence in HD, we reviewed 120 cases before or after treatment. MBCs were identified morphologically and immunohistochemically in 8 cases (nodular paragranuloma, n = 2; nodular sclerosis, n = 2; and interfollicular mixed cellularity HD, n = 4). Acute toxoplasmic, cytomegalovirus, or Epstein-Barr virus (EBV) infections were excluded by serological tests and immunohistochemistry. MBCs were negative by immunostaining for EBV encoded latent membrane protein, while Sternberg-Reed and Hodgkin's cells expressed positivity in 50% of cases. MBCs were only identified in cases with partial or incomplete lymph node infiltration by HD together with an activated B-zone of residual non-infiltrated tissue. The relation of MBCs and HD infiltrates followed three distinct patterns: large HD infiltrates without any connection to MBC foci; small areas containing various numbers of Sternberg-Reed and Hodgkin's cells at the border between MBC foci and surrounding lymphoid tissue; and HD infiltrates within at least some MBC clusters. The data obtained suggest that MBCs occurring in HD represent a transient phenomenon associated with a B-zone activation irrespective of treatment and that they are usually not histogenetically related to HD.