We examined the possibility that the common cold or afebrile upper respiratory tract infection might interfere with successful immunization in children who receive standard measles-mumps-rubella vaccine. Infants 15 to 18 months of age presenting at our well-child clinics for routine examination and immunizations were divided into two groups. Those infants with a history and physical findings of upper respiratory tract infection were compared with healthy control group infants who did not have upper respiratory tract infections, and who did not have a history of upper respiratory tract infection symptoms within the previous month. Both groups were studied for their serologic response to measles-mumps-rubella vaccination. Prevaccination serum samples were obtained prior to vaccine administration and postvaccination serum samples were obtained 6 to 8 weeks later. Measles antibody was measured in these serum samples by an indirect fluorescein-tagged antibody test. Ten (21%) of 47 infants with colds failed to develop measles antibody, while only one (2%) of 51 well infants failed to develop antibody. We conclude that infants with colds have a significant seroconversion failure rate associated with measles vaccine administration and that this may be the cause of some primary measles vaccine failures.