An investigation was performed after an outbreak of bartonellosis in a region of Peru nonendemic for this disorder. Symptoms of acute and chronic bartonellosis were recorded. Serological analysis was performed on 55% of the affected population (554 individuals), 77.5% of whom demonstrated previous infection with Bartonella bacilliformis. The attack rate of Oroya fever was 13.8% (123 cases); the case-fatality rate was 0.7%. The attack rate of verruga peruana was 17.6%. A new specific immunostain was developed and used to confirm the presence of B. bacilliformis in the biopsied skin lesions. Most seropositive individuals (56%) were asymptomatic. The symptoms that were associated with prior infection, as determined by Western blot, included fever (37.2% of the seropositive vs. 17.2% of the seronegative population; P<.001), bone and joint pain (27% vs. 9%; P<.001), headache (27% vs. 12.3%; P <.001), and skin lesions described as verruga peruana (26.8% vs. 4.9%; P<.001). Our findings suggest that infection with B. bacilliformis causes a broad spectrum of disease that is significantly milder in severity than that frequently reported.