A failure assessment method of stay cables containing fractured wires is presented through an example of an existing bridge. The wires constituting the stay cables of the bridge contained many initial defects that were formed during the rolling process of these wires. Despite moderate loading, fatigue cracks started to grow from these defects, increasing their size, and finally resulting in fracture of the remaining ligament of the wires. A probabilistic model is put forward for assessing the resistance of the stay cable based on a fracture mechanics model of the individual wires. A Bayesian update method is presented for estimating the distribution of initial defects based on the inspection of fractured wires. The calculated reliability of the stay cables of the examined bridge was lower than acceptable.