Purpose ISO 9000 certification has a growing importance for companies, mainly in Europe but also in other countries. Several papers have analysed the effects of ISO 9000 certification on company results, with contradictory conclusions. The purpose of this work is to clarify these possible effects. Design/methodology/approach This paper reports new findings on the topic following a study using data from 713 companies and using the two methodologies that have been applied in the scientific literature: the comparison of certified and non‐certified companies and a longitudinal study of the results of certification. Findings The data suggest that not only is ISO 9000 positive for companies but also it can actually reduce benefits and profitability. Practical implications Several reasons for these results can be suggested and constitute a critique of the almost compulsory character of ISO 9000 as a norm for performance in some markets. Companies should be aware that implementing ISO 9000 just for compliance does not constitute a competitive advantage. Originality/value This research uses objective measures of performance and combines two methodologies: the comparison between certified and non‐certified companies and a longitudinal analysis.