In conjunction with a pertussis vaccine efficacy trial in Germany, nasopharyngeal specimens were collected from May, 1992, to March, 1993, from patients with cough illnesses. Clinical data were obtained by initial and follow-up questionnaires. Bordetella parapertussis was isolated from 38 patients (mean age, 3.5 years; 68% girls). Clinical characteristics in these cases were compared with those of 76 patients (matched by age and sex) with illness caused by Bordetella pertussis during the same period. Findings were: (B. pertussis/B. parapertussis): cough > 4 weeks 57%/37% (P = 0.06); whoop 80%/59% (P = 0.07); whoop > 2 weeks 26%/18% (P = 0.05); paroxysms 90%/83% (P = 0.5); body temperature > or = 38 degrees C 9%/0% (P = 0.17); vomiting 47%/42% (P = 0.69); and mean leukocyte and lymphocyte counts 12,500/mm3 and 7600/mm3 (P < 0.0001) and 7800/mm3 and 3500/mm3 (P < 0.0001), respectively. Illness caused by B. parapertussis was typical of pertussis but less severe than that caused by B. pertussis. In contrast with B. pertussis infection, lymphocytosis is not a characteristic of B. parapertussis infection. This is most likely a result of the lack of production of lymphocytosis-promoting factor toxin by B. parapertussis.