BACKGROUND: It is considered to be a fundamental ethical premise of human experimentation, that it should be carried out only where the effects of an intervention are unclear. The point at which it is considered that there is insufficient scientific and medical evidence to clearly state the superiority of an intervention has been termed equipoise. This concept has been the subject of much recent impassioned debate but little empirical research about the views of people involved in recruitment to randomized controlled trials (RCTs), and none in the particularly emotive area of neonatal intensive care. METHODS: Thirty neonatologists recruiting into one or both of two neonatal RCTs in five centres in England were interviewed using a semi-structured schedule to explore their involvement in randomised trials. The interviews were tape-recorded and transcribed. Equipoise was one among a range of topics covered. Concepts relating to equipoise were identified by close reading of the entire interviews. Themes emerging from the data were noted in their contexts then discussed between the co-authors. Interviewees also completed a brief questionnaire about their demographic background, and their experience of research and RCTs. RESULTS: Almost all the neonatologists used the concept of equipoise [using words and phrases such as uncertainty, lack of knowledge (or ignorance), strengths of views, and balancing of pros and cons] in their interview and, for most of them, equipoise seemed to be a useful term. They explored ideas about equipoise at the individual and community levels, and some linked equipoise with notions of the responsibility that should be exercised by the scientific and professional communities. They differed in the importance they gave to individual equipoise, and in how they reacted to threats to equipoise. Feelings of doubt about a trial and disturbed equipoise were more often expressed by more junior doctors. CONCLUSIONS: Our findings suggest that the concept of equipoise goes beyond the idea of uncertainty. In part this is because it includes the balancing of benefit and harm; this balancing is part of a professional obligation and requires engagement with 'expert' knowledge. Equipoise could therefore be seen as 'active' or 'responsible' uncertainty. Elucidation of this difficult concept may help to facilitate recruitment for both clinicians and parents in future trials and thereby help to find answers to important clinical questions.