Clinical and bacteriological findings in the diagnosis of pertussis were evaluated in 300 consecutive patients with parental or the patient's own suspicion of the disease. Serology was used as a reference method. Of the 285 (95%) patients fully sampled, 163 (57%) were diagnosed as having pertussis while the remaining 122 patients constituted the non-pertussis control group. The clinical and epidemiological data were collected at the first visit made on median day seven of illness. In this population of mainly unimmunised children, the highest predictive values were obtained for the physician's diagnosis of pertussis (100%) and for the physician's diagnosis of some other illness (93%). The only clinical symptom with a high predictive value for pertussis was the report of whoops (92%). Among epidemiological data the highest predictive value (90%) was obtained for reported household exposure in unimmunised children more than 1 year of age. Culture of Bordetella pertussis was found to have an overall 50% sensitivity. Isolation of other bacteria had no predictive value in the differential diagnosis of pertussis.