A new model is proposed to describe the effects of structural inhomogeneities in fibrous filters on pressure drop and particle collection. The filter is subdivided into a series of elements with different structural characteristics, defined by local directionally-dependent permeabilities, and the resulting three-dimensional flow field is calculated. Using published relationships, local particle collection is calculated. Spatially nonhomogeneous fibre distribution is shown to play a dominant role. The difference in fibre collection efficiency between a real and a homogeneous filter depends on the dominant mechanism transporting particles to the fibres. In the interception-inertia range, the predictions are verified quantitatively by measurements on industrial filters with characterised structure.