The contradictory effect of lean and industry 4.0 synergy on firm performance: a meta-analysis Diego Biondo Industrial and Systems Engineering Graduate Program, Polytechnic School, Pontifical Catholic University of Parana (PUCPR), Curitiba, Brazil and Serdia Eletronica Industrial, Curitiba, Brazil Dalton Alexandre Kai Industrial and Systems Engineering Graduate Program, Polytechnic School, Pontifical Catholic University of Parana (PUCPR), Curitiba, Brazil Edson Pinheiro de Lima Federal University of Technology Parana, Pato Branco, Brazil, and Guilherme Brittes Benitez Industrial and Systems Engineering Graduate Program, Polytechnic School, Pontifical Catholic University of Parana (PUCPR), Curitiba, Brazil Abstract Purpose – While previous operations management literature acknowledges the positive influence of Lean and Industry (I4.0) on performance, recent studies examining the synergy between these two factors have produced inconsistent and contradictory results. Therefore, this study aims to provide a comprehensive understanding of the effect of Lean and I4.0 synergy on firm performance. Design/methodology/approach – This study utilised a meta-analysis approach, examining 23 empirical studies exploring multiple effects of the Lean and I4.0 synergy on firm performance. Multiple subgroup analyses were conducted to assess the contradictory outcomes and identify in what conditions such synergy may achieve performance. Findings – The results affirm the prevailing positivist perspective among most scholars regarding the positive influence of the Lean and I4.0 synergy on firm performance. However, the overall effect size derived from the studies indicates a weak relationship, suggesting that this synergy alone is not the sole determinant factor of firm performance. In addition, the subgroup analyses reveal the presence of contingent conditions that may affect the performance outcomes when integrating Lean and I4.0, as most effects exhibit a weak relationship. Originality/value – This study represents the first meta-analysis investigating the relationship between the Lean and I4.0 synergy on firm performance. By shedding light on the contradictory effects often depicted in the operations management literature, this study provides a critical reflection for researchers who tend to adopt an overly optimistic view of such synergy. Keywords Meta-analysis, Lean, Industry 4.0, Firm performance Paper type Literature review Quick value overview Interesting because: The existing literature presents an optimistic view of the integration of Lean and Industry 4.0 (I4.0) concerning firm performance. Numerous studies posit a positive Lean and industry 4.0 synergy This project was financially supported by the Education Incentive Policy of the Marist Group’s Individual Development Plan, which includes the Pontifical Catholic University of Paran a as a member institution and by the Research Coordination of the Brazilian Ministry of Education – CAPES. We also thank the National Council for Scientific and Technological Development (CNPq) for financial support in project no. 40355720232. The current issue and full text archive of this journal is available on Emerald Insight at: https://www.emerald.com/insight/1741-038X.htm Received 8 October 2023 Revised 21 January 2024 13 February 2024 Accepted 13 February 2024 Journal of Manufacturing Technology Management © Emerald Publishing Limited 1741-038X DOI 10.1108/JMTM-10-2023-0447