Two groups of six cats were established, one a control group and one infected with feline immunodeficiency virus (FIV) 18 months previously. The cats in both groups were inoculated with Chlamydia psittaci and the clinical progression of the infection was monitored by means of a clinical scoring system for 10 months. Haematological, serological and viral and chlamydial isolation studies were also made. The response of the FIV infected group to treatment with oxytetracycline was monitored in the 11th and 12th months. The FIV infection prolonged the duration of the clinical signs resulting from the infection with C psittaci and led to the development of chronic conjunctivitis. The haematological and antibody responses to C psittaci were comparable in the two groups. However, it was possible to isolate C psittaci from the FIV-infected cats up to day 270, when the treatment began, but only up to day 70 in the control group. In addition, it appeared that the infection with a secondary pathogen may have accelerated the clinical progression of the FIV infection.