Chlamydia pneumoniae infection is said to be associated with obesity. We studied the association between C. pneumoniae infection and inflammation and increased BMI in 891 Finnish military recruits. IgG seropositivity in arrival and departure serum samples during 6–12 months of military service was considered as persistence of antibodies and a possible indication of chronic infection. Persistently high C-reactive protein (CRP) level (elevated on arrival and departure) (OR 2·2, 95% CI 1·3–3·9), and persistent C. pneumoniae antibodies (OR 2·1, 95% CI 1·5–2·8) were significant risk factors for overweight (BMI ⩾25 kg/m2). In addition, those who had persistent antibodies and persistently elevated CRP levels, or those who had either of them, had a significantly higher BMI (kg/m2) compared to those who had neither of them (25·8 vs. 24·6 vs. 23·5, respectively; P<0·001). These results provide new information about the association between possible chronic C. pneumoniae infection and obesity in young men.