SummaryIs diphtheria avoidable?Inoculation against diphtheria has in the past been practiced with different intensity in different parts of the country; but a direct comparison of percentages of the population falling ill does not provide any trustworthy information as to the effect of inoculation, because an epidemic does not reach its peak everywhere i the same year, and because marked differences in morbidity are known to exist between dgferent groups of the population quite apart from the effect of inoculation.It was observed, however, that inoculation was mainly applied to persons between 5 and 14 years of age, and this fact has been used as a basis for investigating its efficiency. To this end two indices were employed, viz:I1=number of cases in the group below 5 years/number of cases in the group above 14 years andI2number of cases in the group from 5 to 14 years/number of cases in the groups below 5 and above 14 yearsThese were computed separately for different boroughs; I1over a period of about 20 years from I924 onwards, and I2, over the two epidemic years I942 and 1943. The data mentioned in this article refer to 1943 only.The index I1is largely independent of inoculation, because few persons in both the age groups considered were inoculated. Hence I1enables us to investigate what social factors influence the distribution of diphtheria cases amongst dgferent age groups. It was found that density of population and family size were the two main influencing factors, which between them could explain by far the greatest part of the variation in I1(multiple correlation coefficient of 0.94).The index I2depends on inoculation, because inoculation is mainly practiced on the group occurring in the numerator. Hence by comparing the index I2for boroughs where the degree of inoculation was ‘good’, ‘medium’, and ‘bad’, we obtain information concerning the effect of inoculation. The results obtained with I1made it possible, however, to group the boroughs in such a manner that the disturbing effect of population density and family size could be in a large measure eliminated, so that the effect of inoculation could be estimated with much greater certainty.This effect turned out to be highly significant.It is estimated that by regular inoculation the risk of falling ill with diphtheria can be reduced by about 61%, and the risk of death by 78%, while various arguments suggest that these estimates must be too low, and that the real effect of inoculation is higher still. Whether it will be possible by systematic inoculation of the whole population altogether to eliminate diphtheria cannot positiuely be decided, but can not be excl