There is no approved design procedure for the design of large-diameter laterally loaded monopiles in sand, e.g. monopiles used as foundations for offshore wind turbines. Recently installed monopiles have diameters of 4-6 m and embedded pile lengths of 18-30 m. The p-y curve method, given in offshore design regulations, is usually employed for the design of monopiles. However, this method was developed for slender piles with diameters much less than 6 m and it is based on a limited number of tests. The aim of the present work is to extend the p-y curve method to large-diameter non-slender piles by considering the effects of the pile diameter on the soil-pile interaction. The main focus is the initial stiffness of the p-y curves which, according to the current offshore design regulations, is governed by the initial modulus of subgrade reaction, k, multiplied with the depth measured from the soil surface. The initial modulus of subgrade reaction is according to the design regulations determined on basis of the relative density or the angle of internal friction, hence, it is considered independent of the pile properties. The evaluation of the soil-pile interaction for large-diameter piles is based on experimental work as well as three-dimensional numerical analyses. Prior to the analyses, a consistent review concerning shortcomings and advantages of the currently recommended p-y curves was conducted. Considering the effect of diameter to the initial stiffness of the p-y curves contradictory conclusions has been drawn through time. A predominant part of researchers conclude that the effects of diameter are negligible. However, most of the analyses are conducted on diameters far smaller than the diameters employed for offshore wind turbine foundations. Furthermore, it is found that the non-slender monopiles behave as almost rigid objects when subjected to lateral loads. Hence, the failure mode is different from the one presumed in the existing p-y curve method. These findings are employed as basis for the experimental work and numerical analyses. The numerical analyses are made by means of the commercial programs FLAC 3D and Plaxis 3D Foundation. In both models a Mohr-Coulomb material model is employed. The numerical models are validated through six small-scale tests of heavily instrumented piles with diameters varying between D=60-80 mm subjected to lateral loads acting with a given vertical eccentricity. Both test piles have a slenderness ratio of L/D=5 implying embedded pile lengths of L=300-400 mm. The tests are successfully carried out in a pressure tank at different effective stress levels in order to overcome sources of error, such as; small non-measurable strains, non-linear failure criterion, and excessive angles of internal friction. After validating the models to small-scale tests the numerical models are extended to full-scale offshore wind turbine foundations with diameters of D=[2,3,5,7] m. The results are compared to results obtained from a traditional p-y curve design based on a Winkler model approach.