In this work we present the first results on a 'label-free' DNA analysis system based on hydrogenated amorphous silicon (a-Si:H) thin film sensors. The detection mechanism is based on absorbance measurements performed on DNA molecules in a buffer solution. The experiments made with single-stranded 30-mer DNA at different concentrations showed an excellent linearity over three orders of magnitude. The extrapolated minimum quantity (expressed as concentration - optical path product) detectable by our set-up was 2 nmol×cm, limited by short term variations of the UV source intensity. Furthermore, relying on the hypo-chromic effect it is possible to distinguish between single- and double-stranded DNA chains. This was observed performing DNA hybridization experiment on a functionalized glass substrate.