The coverage of children by immunization against diphtheria, pertussis and tetanus during the first 3 years of life as indicated in forms No. 112 and No. 63 in 4 districts of Moscow was studied on the basis of the analysis of 1688 children's development histories. The coverage of children by immunization was found to be on the average 71.7%, fluctuating in individual districts from 58.6% to 85.9%. Immunization against measles covered 41.8% of children with fluctuations 24.2-51.2%. The most frequent reasons for the absence of prophylactic immunization in children during the first 3 years of life, or belated immunization in comparison with the specified terms, were contraindications due to neurological diseases (40.4%), exudative diathesis (15.9%) and recurrent viral infections (16.2%). Medical objections to immunization without sufficient grounds constituted 12.1% of all contraindications; among such objections those given by neuropathologists (9.8%) and pediatricians (2.3%) prevailed. To increase the coverage of children by immunization in the specified terms, the use of such measures as improving work with parents (explaining to them the importance of timely prophylactic immunization) and strict observance of instructions on medical contraindications may be considered most effective.