Flexible polyurethane foam based on methylene diphenyl isocyanates and isocyanate-reactive compounds selected from polyols and polyamines, for which the ratio of the density expressed in kg/m.sup.3 to the compression hardness at 40% compression, expressed in kPa, has a value of at least 20.Such foams may be obtained through a process which comprises reacting methylene diphenyl isocyanates with one or more isocyanate-reactive compounds selected from polyols and polyamines with a functionality of 2-4 and an average equivalent weight ranging from 750-5000 and from 0 to 5 parts by weight of water per 100 parts of polyols and/or polyamine, optionally together with additives conventional in the art, and optionally together with blowing agents other than water, whereas the isocyanate index ranges from 10 to 60.