Chlamydial infection of the human respiratory tract was first described over 100 years ago, when outbreaks of psittacosis were linked to imported pet birds. The causative organism was identified subsequently as Chlamydia psittaci. However, C. pneumoniae, has also been recognised recently as an important cause of human respiratory tract infection. Approximately 300 cases of human chlamydial respiratory tract infection are reported to CDSC each year, mainly in adults aged 15 to 44 years. A prospective study in Cambridgeshire showed that 76% of cases were associated with C. psittaci and 24% of cases with C. pneumoniae. There are several laboratory tests for diagnosing chlamydial infections. The complement fixation test is still the most commonly used but it cannot distinguish between different species of chlamydiae. Other serological tests, which can distinguish between different species, are only carried out in specialist laboratories. C. pneumoniae is particularly difficult to culture and C. psittaci is a grade 3 pathogen. Chlamydial antigens can be detected in sputum samples but these are not routinely sought in human chlamydial respiratory infections. There is a need to improve diagnostic techniques because rapid and accurate diagnosis leads to the use of appropriate antibiotics and will help to elucidate the epidemiology of these infections.