Campylobacter upsaliensis was isolated from the feces of 20 human immunodeficiency virus (HIV)-infected patients with diarrhea over a 67-month period, representing 18.5% of fecal Campylobacter isolates from our HIV-seropositive patients. All isolates were recovered from feces by a 0.45-micron-membrane filtration method. Culture on Skirrow's selective medium incubated at 42 degrees C failed to support any growth. The mean CD4 cell count of the patient group was 185/mm3 (range, 0-840/mm3), and all patients had diarrhea for 1-60 days (mean, 19 days). Thirteen had no other enteric pathogen isolated, and three patients (CD4 counts, < or = 70/mm3) had persistent carriage for between 5 weeks and 3 months. All tested isolates were susceptible to erythromycin and doxycycline, but three isolates from two patients were resistant to ciprofloxacin. C. upsaliensis is associated with prolonged diarrhea of mild to moderate severity in HIV-infected patients.