The association of premature rupture of the membranes (PROM) and ascending infection was investigated in 15 women. Bacteriological, histological and immunofluorescence methods were used to study infection at various levels in the birth canal. In most of the women the membranes as well as the placentae showed heavy bacterial invasion. Bacterial distribution within the membranes showed a choriodecidual preponderance. Ascending infection appears to follow the choriodecidual route and may be a primary pathogenetic event in many instances of PROM. The anaerobe Bacteroides fragilis, which is very infrequently isolated in normal pregnant women, was found in five out of 15 women. Two infants had congenital pneumonia caused by group B streptococci and Haemophilus influenzae respectively. The neonatal outcome with PROM may be influenced by the efficiency of the individual defence mechanisms including the antimicrobial capacity of amniotic fluid.