Defective or immature antibody responses to pathogens in children may explain the increased susceptibility to acute otitis media (AOM) in otitis-prone children. In literature, data on immunology have been based on studies of small groups of severely otitis-prone children and have not been consistent. Humoral immune status was assessed in 365 children, 1-7 years old, with two or more documented episodes of AOM in the previous year. Children with 4 or more episodes in the preceding year were defined as otitis-prone. Serum immunoglobulin levels were determined by radial immunodiffusion. Immunoglobulin levels of otitis-prone children were compared with those of children who had experienced 2-3 AOM episodes per year. Children with recurrent episodes of AOM were found to have normal or increased serum IgA, IgM, IgG, and IgG1 levels compared with normal values for age, whereas the serum IgG2 levels were mostly in the lower normal range. Twenty-two percent of all children showed IgG2 levels lower than 2 SD below the age-specific mean. Interestingly, the otitis-prone group of children showed significantly lower median and mean levels for all immunoglobulins compared with those children with only 2-3 previous AOM episodes. Lower immunoglobulin levels in otitis-prone children suggest a generalized decreased antibody response in otitis-prone children.