The origin of contamination in pertussis of young infants is generally the close relatives. From 2000 to 2004, only serology and culture were available in our hospital. The families of 16 young infants (age below one year) hospitalized for pertussis were screened using serological tests: 21/48 contacts were positive. After 2004, PCR was available for exploration of index cases and families: 35/85 contacts were positive. Of the mothers tested 23/46 were positive compared to 14/41 fathers. Only one parent presented with a typical paroxystic pertussis cough, 60% presented with a nonparoxystic cough having lasted for more than five days and 40% of positive adults did not present with cough. Despite official recommendations, none of these young parents had received an antipertussis booster vaccination. This study shows the high frequency of atypical or nonsymptomatic pertussis in adults in the close family of infected young infants. These adults contribute to spreading the disease.