Anti-rubella IgG titer was estimated in 106 children: sixty five had acute rheumatic fever and 41 had either a history of rheumatic fever or a definite rheumatic heart disease. Their ages ranged from 6 to 16 years old (11.4 +/- 2.7 years). Sixty two were females and 44 were males. They were compared with 35 control normal subjects of both sexes with no history of rheumatic affection, aged 5 to 14 years (10.2 +/- 3.6 years). The results obtained showed a significantly (p less than 0.001) higher rubella IgG titer level in the controls (1066.5 +/- 8.3) as compared to the rheumatic cases (521.2 +/- 3.0). No differences were found in the rubella antibody titer between cases of acute rheumatic fever and those with history of rheumatic fever or rheumatic heart disease, nor between males and females in either cases or controls. We suggested that the low rubella IgG levels encountered in the rheumatic cases, make them more susceptible to rheumatic fever affection. In other words, a high immunity level against rubella might also produce an immunity against an agent, possibly related to rubella virus, with a specific role in the development of rheumatic fever. Vaccination against rubella could be worth a trial, for prevention of rheumatic fever.