The incidence of natural poliomyelitis in hypogammaglobulinemics has been estimated to be similar to that in nonimmune normal children of the same age group exposed to virulent poliovirus. Poliomyelitis that afflicts hypogammaglobulinemics after the feeding of oral polio vaccine is characterized by an incubation period of longer than 28 days, a high rate of mortality after a long chronic illness, abnormal lesions in the central nervous system, and no reversion of the vaccine virus to virulence. Nearly 10% of vaccine-associated cases are in hypogammaglobulinemics, who are estimated to be 10,000 times more susceptible to vaccine-induced poliomyelitis than normal persons. It is suggested that only inactivated poliovaccine be given to hypogammaglobulinemics, that all suspected cases of vaccine-induced poliomyelitis be tested for γ-globulin status, and that persons be tested for antibodies to poliovirus before treatment with immunosuppressive drugs.