Silent intrauterine infection as cause of preterm labour should be recognised early by measuring C-reactive protein (CRP) levels in the maternal serum. Ensuing antibiotic therapy may not only cause gestational prolongation, but also has a positive effect on pathology during the puerperal period. Consequently, we examined the rate of infectious puerperal complications of 181 patients who suffered from so-called idiopathic preterm labour. 84 patients showed normal CRP-levels at the onset of tocolysis (< 5 mg/l); an elevated level was found in 97 patients (> 5 mg/l). Patients with elevated CRP-levels and supplemented antibiotic therapy during pregnancy (n = 51) showed significantly fewer symptoms of postpartal endometritis than women with similarly elevated CRP-levels but without antibiotics (n = 46). This effect was not produced when preterm labour was associated with normal CRP-levels. Patients with premature contractions and elevated CRP-values at the onset of tocolysis seem to benefit from the antibiotic therapy during pregnancy because they show reduced infectious complications throughout the puerperal period.