Adrenomedullin (AM) gains its bioactivity by amidation at its C-terminal, forming "mature AM." The mature AM and the expression of AM receptor component mRNAs, receptor activity-modifying protein 2 and calcitonin receptor-like receptor, from feto-maternal tissues of normal pregnant women and women with histologic chorioamnionitis were examined to clarify the pathophysiological features of this intrauterine infection. Samples of the placenta and fetal membranes were obtained from 10 normal pregnant women and eight women with histologic chorioamnionitis under informed consent. Mature AM in the fetal membranes was significantly lower in patients with chorioamnionitis than in normal pregnant women. On the other hand, there were no differences in mature AM levels in the placenta between the two groups. The total AM levels as a sum of mature and immature AM were not significantly different between the two groups in either area. The ratio of mature AM/total AM was significantly decreased in the fetal membranes of the patients with chorioamnionitis compared with normal pregnancies, but not in the placenta. Also, levels of mature AM were negatively correlated with C-reactive protein concentrations. The present results thus suggested that mature AM may have some role in chorioamnionitis.