OBJECTIVE Granular lymphocyte proliferation and neutropenia with or without splenomegaly occurs with unknown frequency in rheumatoid arthritis. We decided to evaluate the prevalence of Felty's syndrome and granular lymphocyte proliferation among patients with rheumatoid arthritis and to determine the fraction of patients with granular lymphocyte proliferation who also had rheumatoid arthritis. RESULTS We retrospectively analyzed 1,053 cases of rheumatoid arthritis and 13,505 marrow examination reports for the decade 1978 to 1987. Among patients with Felty's syndrome rheumatoid arthritis with neutropenia/leukopenia, and rheumatoid arthritis with splenomegaly, we identified 18 patients with neutropenia as a manifestation of rheumatoid arthritis. We also identified marrow examinations in 150 patients with rheumatoid arthritis. Using blood counts, microscopy of marrow, and surface antigen analysis of mononuclear cells, we determined that 12 patients had typical Felty's syndrome and six had granular lymphocyte proliferation, representing prevalences of 1.1 percent and 0.6 percent, respectively. No patient had granular lymphocyte proliferation without neutropenia. CONCLUSIONS Granular lymphocyte proliferation and neutropenia with or without splenomegaly in rheumatoid arthritis commonly resembles typical Felty's syndrome. Further, the six patients with granular lymphocyte proliferation represent 20 percent of our institution's patients with granular lymphocyte proliferation, supporting the previously described common association of this disorder with rheumatoid arthritis. The relatively large fraction of deaths (due to malignancy and infection) among the patients with typical Felty's syndrome suggests that their mean survival may be comparatively less than in those with granular lymphocyte proliferation.