In current design of offshore wind turbines, monopiles are often used as foundation. The behaviour of the monopiles when subjected to lateral loading has not been fully investigated, e.g. the diameter effect on the soil response. In this paper the diameter effect on laterally loaded non-slender piles in sand is evaluated by means of results from six small-scale laboratory tests, numerical modelling of the same test setup and existing theory. From the numerical models p-y curves are conducted and compared to current design regulations. It is found that the recommendations in API (1993) and DNV (1992) are in poor agreement with the numerically obtained p-y curves. The initial stiffness, Epy*, of the p-y curves, is found to be dependent on the pile diameter, i.e. the initial stiffness increases with increasing pile diameter. Further, the dependency is found to be in agreement with the suggestions in Sørensen et al. (2010). It is found that considerable uncertainties are related to small-scale testing, and the different evaluations clearly indicate that the accuracy of small-scale testing is increased when increasing the pile diameter and applying overburden pressure.