In March 1969, an outbreak of upper respiratory illness occurred among 4930 new military trainees at the San Clemente Sasebas Camp in northeast Spain. The illnesses were clinically mild, consisting mostly of febrile pharyngitis, cough, and myalgia, but they spread explosively throughout the camp to affect 65% of the recruits in March. For the illnesses of 25 men selected at the midpoint of the outbreak, adenovirus type 11 and influenza virus A were the principal etiologic agents and adenovirus types 3 and 7a were involved to lesser extents. The influenza virus infections appeared to follow closely the adenovirus infections in cases of double or sequential infection. The adenovirus 11 strains were atypical in that they failed to agglutinate monkey erythrocytes. This is the first report of the isolation of adenovirus 11 from throat specimens and the association of type 11 with upper respiratory illness, particularly in military recruits.