A recent strand of the economic literature has emphasised the role of services, and in particular knowledge-intensive business services (KIBS), as a primary source of knowledge creation and diffusion. Since this transferring process often occurs through strong face-to-face interactions, the role of spatial proximity becomes crucial. Theoretical and empirical literature shows that the geographic concentration of industry induces firms to vertically disintegrate their production, due to the lowering of transport and governance costs as well as to the reduction of opportunism in managing transactions. However, the evidence is primarily based on manufacturing firms, whereas little or no attention is given to service firms. In this paper we try to fill this gap by estimating the effects of spatial agglomeration on knowledge intensive business service firms' vertical disintegration with reference to the metropolitan region of Milan. Relying on a rich firm-level dataset of about 12.000 KIBS firms located in the metropolitan area of Milan in 2008, we first geo-referenciate our data by employing a GIS routine. Then, we define a set of rings moving out of increments of 1 kilometre, and we count the number of firms located within each ring. For each firm, we compute, ring by ring, the number of neighbouring firms that are in the same three-digit industry, and the number of firms that are in all the three-digit industries except for the one in which the firm operates. In this way, we estimate the impact of proximity-based specialization Vs diversification economies on KIBS firms' vertical disintegration, exploiting information on the actual distance between each pair offirms in the sample. Our dependent variable is calculated as the share of purchased business services over total production costs. This purchased-inputs variable allows accounting for the fact that "many business services are likely to be exactly the kind of locally produced intermediate input that producers in localized areas will have greater access to than producers in isolated areas" (Holmes 1999, p. 316).