ESDU 85037 gives information on a Fortran program available on disc (uncompiled) in the software volume as ESDUpac A8537 for estimating the lower natural frequencies of initially unstressed rectangular sandwich panels. It is presented for convenient use when the face plates are fibre-reinforced laminates. It applies to flat@ or slightly curved@ panels with thin orthotropic face plates having their principal axes of orthotropy aligned with the sides. The face plates may be of different materials and thicknesses and the core is assumed to have zero flexural stiffness. Simply-supported edges are assumed@ but guidance is provided on the use of effective panel dimensions to allow consideration of other edge conditions. Predictions of natural frequency are compared with test results from the literature for panels with fibre-reinforced face plates and agree to within 10 per cent. Details of input and output format are provided through a practical worked example.