Among 96 rural residents of Puerto Rico who were evaluated in groups of 10 to 15 per month during the 8-month period of July through February, the prevalence of malabsorption of single and multiple substances and of severe jejunal morphologic abnormalities was significantly higher during the months of December through February than during the preceding 5 months. The majority (61%) of persons with overt tropical sprue seen at this Unit over a 1-year period also experienced the onset of gastrointestinal symptoms during this 3-month winter period. In contrast, no such seasonal variation was evident among 16 expatriates from the West Indies who developed their initial symptoms of sprue while resident in New York City. Enterotoxigenic coliform bacteria were cultured from the jejunum in 10 of 11 persons with tropical sprue who were investigated throughout the year; these organisms were also present in 2 of 3 rural residents examined in December-February, but in none of 6 other persons studied during the preceding 4 months. These observations point towards an environmental factor that is present in Puerto Rico during the period of December through February which is associated with a high prevalence of intestinal abnormalities in the general population, the onset of symptoms in persons with tropical sprue, and colonization of the small intestine by enterotoxigenic bacteria. It is suggested that changes in dietary lipid that occur during the month-long Christmas season in Puerto Rico may be of relevance by virtue of affecting the intestinal milieu in a manner favorable for coliform growth.