Summary A serological diagnosis of ornithosis was made in 24 patients attending chest clinics in south Somerset during 1964 and 1965, 16 being discovered in one year of routine testing of all (156) patients referred after a ‘respiratory influenzal’ illness. A persistent dry, troublesome cough was a prominent feature in all but 2 patients; and clinically the disease imitated variously pertussis, a slowly-resolving pneumonia, carcinoma and tuberculosis. Pneumonic radiographic changes, usually basal and minimal, were seen in 15 patients. The illness was protracted in 8 instances, 5 hospital admissions being necessary. Half the 22 patients given tetracycline appeared to derive some benefit. Except in 5 relapses, the CFTs fell to 1/40 or less within a year of diagnosis. Close avian contact was recorded in 18 cases. Examination of 71 contacts of our cases showed that 53% had positive CFTs (at a 1/10 or greater dilution) compared with a figure of only 6% in comparable routine investigations of other subjects during the same period. The epidemiological value of serial testing was demonstrated in 11 child contacts with sub-clinical infection. The literature indicates that up to 10% of major virus pneumonias are due to ornithosis: this paper shows that sometimes a similar proportion of less severe respiratory illness may be due to the same infection.