The immunohistologic characteristics of synovial membrane obtained from patients with active rheumatoid arthritis (RA) were correlated with disease activity and spontaneous in vitro synthesis of IgM rheumatoid factor (RF) by peripheral blood mononuclear cells (MNC). Positive correlations were found between the intensity of inflammatory cell infiltration and both disease activity (p = 0.0007) and RF synthesis (p = 0.028). Moreover, the intensity of both T cell and B cell infiltration correlated significantly with disease activity. Within the group studied 3 previously described immunohistologic categories were identified. These categories could not be distinguished by clinical measurements, but did differ with respect to their capacity for spontaneous in vitro IgM RF synthesis by blood MNC. Our study provides evidence that a relationship does exist between the immunohistologic characteristics and disease severity in RA. Furthermore, the concept of distinct histologic categories of RA representing different immunopathogenic mechanisms is supported.