Biological studies of the problem precipitated by the outbreak of illness, and deaths in the Donora area during the smog of October 1948, were concerned principally with an attempt to describe the acute episode in terms of the incidence and severity of illness in the general population, and to develop a clinical picture of the disease. In addition, an investigation was made of the possible effects on health of continued living in the community, since such effects might well point to the existence of atmospheric contamination of a mild nature, which during the period of prolonged smog might have contributed to the acute outbreak.