PIP: A large, randomized, double-blind trial has made available sufficient data to recommend the simultaneous administration of combined measles-mumps-rubella (MMR), diphtheria and tetanus toxoids and pertussis (DTP), and oral poliovirus (OPV) vaccines to all children 15 months old or older who are eligible to receive these vaccines. In this trial, serologic response and clinical reaction rates following primary immunization with MMR were compared in a test group of 405 children given MMR simultaneously with DTP and OPV and a control group of 410 children given MMR followed by doses of DTP and OPV vaccine 2 months later. Seroconversion rates to each MMR component exceeded 96% in both groups, and the geometric mean titers achieved against the other 6 antigens also were similar in both groups. Rates of most of the common vaccine-associated clinical reactions to DTP and MMR were not augmented by simultaneous administration of these 2 vaccines. Some minor side effects were reported more frequently in the simultaneous-administration group, but these difference were judged to be related to artifacts of the study design rather than to differences in the safety of the 2 vaccine schedules. Data from the Center for Disease Control's Monitoring System for Adverse Events Following Immunization have been reviewed, particularly the information from Idaho, Louisiana, and Tennessee, where policies to administer MMR, DTP, and OPV simultaneously have been in effect for periods ranging from several months to years. The evidence suggests no increased risk of reactions associated with the simulaneous administration of these antigens. The overall implications of simultaneous administartion have not been fully defined, but it is anticipated that implementation of this new schedule will result in at least 3 benefits: a decrease in the number of health care provider visits required for immunization during the 2nd year of life, and accompanying decrease in costs, and an increase in the percentage of children who will be fully or partially immunized by 24 months of age. Some health care providers may continue to prefer administering MMR at 15 months followed by DTP and OPV at 18 months, especially for patients who are known to be compliant.