1. In the period 1923–39, more than one-fifth of all deaths in the age group 0-2 years in the Administrative County of London are recorded as having been caused by bronchitis or broncho-pneumonia. It is very unlikely that the returns have been exaggerated by more than 10–15% through the inclusion of deaths from secondary broncho-pneumonia following measles, whooping cough or influenza. In the 8 years in which measles epidemics occurred, the peak of the measles mortality was from 2 to 17 weeks after, and never before, that of the mortality from broncho-pneumonia.
2. The winter rise in mortality from broncho-pneumonia is not associated with any comparable increase in the carrier rates for pneumococci and haemolytic streptococci.
3. The broncho-pneumonia mortality of infants in the Metropolitan Boroughs is strongly correlated with sub-standard housing incidence, social index and percentage below the poverty line. The relatively small value found for the coefficient of regression between broncho-pneumonia mortality and sub-standard housing may be due to the less rigorous winter temperatures occurring in the more overcrowded central areas.
4. Low external temperature appears to be the most important predisposing cause of fatal broncho-pneumonia in infants; their weekly death-rate from this disease in mid-winter is about seven times that in the late summer. This great seasonal fluctuation in mortality is also found for persons over 55 years in the Administrative County, and for persons of all ages in both the urban and rural districts of the Home Counties.
5. The weekly mortalities of infants from broncho-pneumonia in the Administrative County are most strongly correlated with low temperatures 2 weeks earlier. The two variables are so associated that the mortality rises sharply when the mean temperature of the coldest day of the week falls below 40° F. There is no indication that low temperatures in the spring are associated with higher death-rates from broncho-pneumonia than the same temperatures in the autumn, so that the data provide no support for the supposition that susceptibility increases during the winter in consequence of the progressive depletion in the body of protective nutrient substances.
6. Possible measures to reduce the mortality of infants from broncho-pneumonia are discussed.