In a prospective study of meningitis in 100 predominantly preterm infants, Ureaplasma urealyticum was isolated from the cerebrospinal fluid (CSF) of 8 and Mycoplasma hominis from the CSF of 5 babies undergoing investigation of suspected sepsis or treatment of hydrocephalus. U urealyticum was isolated from 6 infants with severe intraventricular haemorrhage and from 3 with hydrocephalus. In 4 babies multiple isolations were made over several weeks. There were clinical features of congenital infection with major neurological impairment in 1 infant infected with M hominis. Diagnosis is difficult because these organisms cannot be seen on gram stain and cannot readily be cultivated on routine bacteriological media, and CSF pleocytosis may be absent. This study, which used appropriate mycoplasmal media, shows that U urealyticum and M hominis are the most common microorganisms isolated from the CSF of newborn infants in a high-risk population.