1. A number of major therapeutic trials in mild hypertension were conducted from 1972 to 1980. With one exception, there was little or no coordination between the trials, although they were dealing with the same problem.
2. A Joint WHO/ISH Mild Hypertension Liaison Committee was set up after most of the trials had started. The objective of the Committee, to promote information exchange and comparisons among the trials, was attained. Certain similarities and disparities between the trials were also identified.
3. Some of the disparities proved useful because they made it possible to analyse the problem from different points of view. Others did not introduce any new elements but, on the contrary, hampered inter-trial comparability.
4. Early standardization of methodological details in such trials is essential, particularly if several studies are to investigate the same problem. This would be possible only if prior agreement were reached concerning research policy, especially in rapidly developing fields of hypertension research.