Patients with pneumonia not responding to treatment with betalactam drugs and patients where an "atypical" etiology is suspected from the beginning, are often given erythromycin to cover mycoplasma and legionella. Erythromycin has also been effective for Chlamydia pneumoniae. If, however, ornithosis is suspected the recommended drug has been tetracycline. Since we noted that several patients had a favourable course on erythromycin despite a final serological diagnosis of ornithosis, we retrospectively studied patients admitted with acute lower respiratory tract infection and a 4-fold titer rise to C. psittaci. We found 35 patients treated with a betalactam drug (n = 12), tetracycline (n = 2), or erythromycin (n = 5) alone, or with a betalactam, which because of non-responsiveness was followed by either tetracycline (n = 4) or erythromycin (n = 12). The data were analysed with survival analysis by a Cox' regression model. There was a significant (p less than 0.001) effect of treatment on the time to defervescence, mainly due to a difference between the erythromycin treated group and the betalactam treated group. We found erythromycin to be at least as effective as tetracycline for treating C. psittaci pneumonia. Since erythromycin has to be used to cover legionella in patients with severe pneumonia when an atypical etiology cannot be excluded, it is an important conclusion that this drug seems to cover C. psittaci as well.