Thomas, D. B. (Johns Hopkins Univ. School of Hygiene and Public Health, Baltimore, Md. 21205), W. M. McCormack, I. Arita, M. M. Khan, M. S. Islam and T. M. Mack. Endemic smallpox in rural East Pakistan. I: Methodology, clinical and epidemiologic characteristics of cases, and intervillage transmission. Amer J Epidem 93: 361–372, 1971.—During one year, 119 cases of variola major, representing 30 outbreaks, occurred in 27 villages of a rural study area in East Pakistan. Nearly one fourth of those afflicted died, the majority during the second week of illness. Pre-exposure vaccination reduced mortality. The case fatality ratio was highest for the nearly completely unimmunized children under age 5, and within this group was higher for those under 1 than from 1 to 4. The density of lesions on the arms and face of unvaccinated cases varied widely; previously immunized cases tended to have relatively sparse lesions. Highest attack rates were noted for the unvaccinated, especially 5- to 14-year-old males, and children not attending school. Most cases occurred in the spring. All were a result of introductions into the study area, primarily by landless adult males who frequently came from jobs in large cities where only 5% of the people in East Pakistan reside. Smallpox was introduced into villages with a frequency directly related to their population size. Selective vaccination procedures based on these findings can reduce the potential for intravillage smallpox spread, and diminish the probability of introductions into rural communities.