The relationship between serum immunoglobulins and the severity and risk of Respiratory Syncytial Virus (RSV) infection in the institutionalized elderly was prospectively assessed during the winter of 1989-1990 at a 591 bed nursing home. Forty RSV infections were identified out of 149 respiratory illnesses by isolation of the virus or by a greater than or equal to 4-fold rise in RSV-specific IgG by EIA. Acute serum RSV IgG levels were similar in those with RSV infection and those with non-RSV illness. Additionally, among the RSV-infected elderly there was no correlation between severity of clinical symptoms and level of acute IgG titers by EIA or virus neutralization. The results of this study suggest that humoral immunity does not play a major role in reducing the risk of infection nor modulating the clinical severity of illness in elderly persons with RSV infections.