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The paper presents a probabilistic model used to investigate the effect of different variables upon the failure probability of hydraulic cylinder housings subjected to fatigue loads. The inputs of the model are: i) an experimental distribution of surface discontinuities; ii) a threshold model obtained by experiments on micronotched specimens; iii) geometric tolerances of the tubes; iv) POD curve of the NDT control. A key input in the proposed model is the surface quality, which is modeled considering the distribution of longitudinal surface discontinuities obtained from experimental measurements. Crack propagation threshold has been modeled taking into consideration the effect of short cracks. The probability of detection of surface inhomogeneities during ordinary non destructive tests have been modeled according to data reported in the literature. Finally the Monte Carlo method has been implemented to assess the reliability of a component in different conditions.