The article discusses the concept of the so-called “reducing upholding enforcement” (validity) of juridical acts in the light of the draft of Book One of the new Polish civil code (published in 2008). In the article an analysis has been carried out of the most important theoretical and practical backgrounds. The main concern is about the scope of the judge’s power to reduce or even revise the term of the juridical act (contract) and its limit. The comparative context of reduction has also been presented. Reduction is one of the ways to avoid nullity of the juridical act, especially contract clause and is one form of partial invalidity. The main function of it is to protect parties from the disadvantageous and sometimes unexpected effects of invalidity, especially those effects for a party protected by the sanction of nullity (“weaker party”). Reducing the scope of the void term leads to the legal effects of a juridical act remaining in force to the extent allowed and valid in the realm of private law (for instance to cut out the part infringing mandatory rules). This paper includes also comparative remarks on the admissibility of such a reduction in chosen legal systems ius civile and common law tradition. Amongst ius civile systems the doctrine of reduction is widely accepted in German law (called Geltungserhaltende Reduktion). In the common law tradition, reduction is rather inadmissible as a principle. There is a restrictive approach for partial invalidity within the doctrine of severance in contract law. Even though the severability of the contract is only partly invalid, it is widely recognised that reducing invalid contractual terms is hard to accept because of the blue pencil rule and prohibition of such interventions in contractual matters. The article discusses exceptions which can be found in some countries (especially New Zealand or Québec law on illegal contracts). The fundamental problem arising in the area of reducing upholding effectiveness is connected with the fine line between reducing the scope of a void term as a part of the juridical act and revising the matter hidden behind the invalid part of the juridical act. In other words, with the limitations of the courts’ discretion in rewriting an invalid clause.