The ISO 9001 Standard – a quality assurance standard for products and services – is well established in the construction industry, which is one of the major industries in the world, including Spain. The objectives of this paper are, on the one hand, to analyse the results of adhering to this standard for construction companies in Spain, and on the other, to examine whether these results are affected by the size of the organization and the seniority of the certification. To this end, a questionnaire was sent to quality managers of 302 companies in the industry, and the responses of 126 valid questionnaires received were treated by descriptive analysis and ANOVA. From the answers gathered is concluded that obtaining the ISO 9001 certification has positive effects leading to benefits which are both “internal” and “external”. Some of the most significant internal benefits mentioned were improvements to the organisational processes, while among the external benefits, commercial aspects, such as better chances of competing on the market, were mentioned most frequently. It was also observed that the size of the organization does not significantly modify the positive effects of adherence to the standard, while the age of the certification itself, has a significant influence on the positive results of the adherence.