This study intends to develop a systematic analysis of legal proceedings in a national jurisdiction in order to clarify how the PP has been interpreted and applied by the courts in the analysis of conflicts associated with serious risks to the environment and to public health. It is also intended to contribute to the debate on when and how to apply precautionary measures, thus seeking also to participate in the search for solutions to a more balanced resource utilization and management and a more sustainable society. In order to evaluate the degree of consistency of the courts’ decisions in relation to comparable risks and their proportionality in relation to the seriousness of hazards, a theoretical framework was developed based on three attributes: level of seriousness of hazards, level of evidence required, level of severity of precautionary measures. A case-study application of this framework was developed for Portuguese courts. The judicial cases where the PP was invoked between 2002 and 2018 were considered and a set of relevant cases were selected for review in the areas of waste incineration, high voltage power lines, dam and wind farm construction. Different positions among the courts in the different judicial bodies were observed, with contradictory arguments in the same judicial case or in similar cases. In order to counter this situation, more explicit legal requirements and criteria for the analysis of uncertain risks and the weighting of interests by area of activity, for the level of evidence required and also for the analysis of the proportionality of decisions in relation to the levels of uncertainty and of seriousness of the hazards are proposed.