A girl was born of a mother who had rubella during the first trimester of pregnancy. The child showed failure to thrive, hepatosplenomegaly, lymphadenopathy, glaucoma, and nerve deafness. She also developed a dysgammaglobulinemia with low serum γM, low to absent γA, and high (19s) γM. She was treated with methyl prednisolone and vinblastine and appeared to improve; but, after an infection at age 3½, the lymphadenopathy returned and γG became undetectable. Peripheral blood cells were normally stimulated by phytohemagglutinin. A lymph node biopsy showed a marked histiocytic reaction pattern. The incidence of "histiocytosis" in congenital rubella is reviewed, and the possible relations between congenital rubella, dysgammaglobulinemia, and histiocytic reactions are discussed. The following hypothesis is suggested: histiocytosis may be a tissue reaction to a persistent (viral) infection in the presence of an altered immune mechanism also caused by that infection.