In a blind study the interlobular interstitial connective tissue in paraffin embedded lung sections from 45 autopsy cases aged under 2 years (23 SIDS and 22 Non-SIDS) were investigated with the aim of determining the collagen type I:type III ratio by means of polarimetric evaluation. Sections were stained with Resorcin-Fuchsin for elastin fibres, Celestine Blue/Mayer's Haematoxylin for nuclear details and with Solophenyl Red 3 BL in saturated picric acid solution for the differential staining of type I and type III collagen fibres for polarization microscopy. The type I fibres were orange and the type III fibres green in colour. The spectral distribution of the coloured polarized light from the sections was determined and peaks were evident at the wavelengths 590 nm for the orange coloured light of type I and 490 nm for the green coloured light of type III collagen. With corresponding filtres the intensities of the orange and green emissions were separately measured at several points adjacent to lymphatic vessels. The ratio collagen I/III, deduced from the ratio of the intensities of orange to green light, was significantly higher in the SIDS-group than in the Non-SIDS-group (a = 0.001) due to the increase in the amount of collagen type I and could indicate an insiduous fibrosis resulting from lymphostasis.