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Effusion cooling by discrete slits and holes in various laminar zeropressure gradient super-and hypersonic boundary layers is investigated using direct numerical simulation. For an adiabatic Mach-6 boundary layer it was found that slits are better than holes due to the lower blowing velocity. Slit blowing causes a destabilisation of 2 nd -mode disturbances, and a complete stabilisation of 1 st modes despite the generated maxima of the spanwise vorticity inside the boundary layer. Hole blowing gives rise to counter-rotating streamwise vortices, with a noticeable laminar-flow destabilisation only for large spanwise hole spacings. Computational aspects such as the performance of the numerical code on the supercomputer NEC SX-8 and optimization of the code are also discussed.