A series of 135 subjects (134 children and one adult) with documented or suspected systemic allergy were prick-tested before a measles, mumps, and rubella (MMR) vaccination. Atopic eczema was documented in 68, asthma in 47, and cow's-milk allergy in 11 examinees; eight children were evaluated because of severe systemic reactions following diphtheria-pertussis-tetanus, measles, or inactivated polio (Salk) vaccinations. In one child, there was only a suspicion of general allergy. The undiluted MMR prick test gave negative reactions in 126 cases (93%). The highest rate of nonreactivity was observed in those with atopic eczema (96%) and in children with asthma (91%) or cow's-milk allergy (82%). All examinees with systemic reactions after other vaccinations also had negative prick-test reactions. A total of 122 (95%) of the 129 examinees were eventually vaccinated with MMR. No untoward reactions developed, except mild generalized urticaria or fever in two vaccinees. We conclude that at least 95% of children with common forms of systemic allergy can be vaccinated safely with MMR and, in general, that allergic diseases should not interfere with execution of the vaccination programs.