The Perinatal Trials Service provides a service to busy clinicians who wish to mount large simple-in-design randomized trials. The trials aim to identify moderate, but clinically useful, effects of promising treatments for the most important problems in perinatal care. A combination of a flexible five-person core resource and a cadre of staff employed to work on specific trials allows a programme of trials to be conducted successfully and efficiently. The Perinatal Trials Service has three main areas of work: prevention and treatment of immaturity and its consequent morbidity; identification and management of the compromised fetus or baby; and prevention and treatment of maternal morbidity. Examples of projects from each area are described.