BOIX R. and GALLETTO V. Innovation and industrial districts: a first approach to the measurement and determinants of the I-district effect, Regional Studies. The paper analyses an exhaustive database of patents granted in Spain between 2001 and 2006 aggregated in a panel of 806 local labour markets classified by seven typologies of local production systems. The analysis shows that Marshallian industrial districts generate 30% of Spanish patents and an innovative output per capita that is 47% above the national average and 31% larger than the manufacturing production systems of large firms. The econometric estimates of a fixed-effects model confirm the existence of an innovation-district (I-district) effect and its size. The I-district effect is mainly related to the presence of Marshallian localization economies. © 2009 Regional Studies Association.