Background The UK Medical Research Council has proposed that complex interventions should be tested in exploratory trials prior to a full-scale trial so as to better define the intervention and test the feasibility of components such as recruitment. It is not clear to what extent this is being done. This study aimed to determine to what extent complex interventions are tested prior to a full-scale trial and whether more or different testing would have led to a different intervention being used in the trial. Methods Email survey of the authors of complex intervention trials published in seven major journals in 2004. Results 72% (50/69) of eligible authors replied. Eight authors did not consider their interventions to be complex. The majority of respondents' complex interventions were tested (34/42): some extensively. Conversely, only 17 of the 34 published reports describing these trials mention testing. Two-thirds (22/34) of those testing their interventions did not believe that more or different testing would have produced a more effective intervention. 31% (13/42) of all authors did believe further testing would have led to improvements. Five respondents mentioned a lack of funding as a reason for not doing more testing. Conclusion Complex interventions are generally tested prior to their evaluation in a full-scale trial, although the amount of testing varies. Testing is often not described in trial reports, which makes it hard to judge whether a trial result could be improved with a better intervention, or whether further work with a different intervention is required.