The incidence of nephropathia epidemica (NE), caused by Puumala virus, among military troops operating in an endemic area of Sweden was investigated. A total of 705 soldiers (age 18-25) involved in field training in three different endemic counties were bled twice within a 6-month interval. Three individuals seroconverted when tested for Puumala virus antibodies by IgG ELISA. Symptoms typical of NE were not recorded in any of the patients. However, mild febrile episodes were recorded in 2 of the 3 individuals. One serologically confirmed soldier with typical NE symptoms was found in another group of 12,000 troops. This soldier fell ill 3 weeks after a 7-day-long field exercise. The present study indicates that military populations are at considerably greater risk of contracting NE when compared to the entire population residing in the same area.