The age and the origin of Chlamydia pneumoniae primary infection in Sahelian areas are unknown. To evaluate the prevalence of C. pneumoniae antibodies, the authors studied the serological status of C. pneumoniae-, C. psittaci- and C. trachomatis-specific antibodies of 50 mother-child couples using the microimmunofluorescence technique. Children were 10 to 34 month-old (18,1 6,2, mean SD), breast-fed and hospitalized with acute respiratory tract infections in Niamey (republic of Niger). Specific C. pneumoniae IgG antibodies were present in 46/48 (95.8%) children, IgA in 32/48 (66.6%). In the mothers, specific C. pneumoniae IgG antibodies were present in 40/48 (83.3%), IgA in 31/48 (64.6%). The specific IgM antibodies were steadily absent in mothers and children. In the mother-child couples, 38/46 (79.2%) had specific C. pneumoniae IgG antibodies and 23/47 (48.9%) had IgA. The prevalence of specific C. pneumoniae IgG and IgA antibodies is very high in this population. These results point out that C. pneumoniae primary infection is very precocious in this paediatric population and is very different from American, Asian and European reports. A mother-child direct contamination during the first months of life in this breast-feeding pediatric population is discussed.