An outbreak of unexplained illness occurred in members of an army reserve unit after field training in an area of New Jersey endemic for Lyme disease. Nine (12%) of the 74 who attended the exercise had serological evidence of Ehrlichia infection, defined as a single rise in titer of antibody to Ehrlichia canis greater than or equal to 1:160 four weeks after training. Two reservists with early serum samples had documented seroconversion, defined by a four-fold or greater increase in titer of antibody to E. canis, with a peak titer of greater than or equal to 1:160. Reservists with serological evidence of Ehrlichia infection were more than three times as likely to report arthralgia, myalgia, headache, appetite loss, nausea, eye pain, and abdominal pain than the other reservists. No reservist with serological evidence of Ehrlichia infection was hospitalized and most had minimal or no symptoms. This outbreak of ehrlichiosis suggests that the usual symptoms of Ehrlichia infection are milder than previously reported and that ehrlichiosis must be considered in symptomatic persons with recent tick exposure.