Laterally loaded deep foundations are commonly analyzed using the Winkler model with the soil-pile interaction modeled through nonlinear springs in the form of p-y curves. Computer programs such as FloridaPier and COM624P use default p-y curves when performing lateral analyses. These curves are based on input soil properties such as subgrade modulus, friction angle, undrained shear strength, etc. Soil properties must be deduced by laboratory testing or correlation to in situ test results. This is a source of uncertainty. In a few cases, lateral load tests are performed on instrumented piles, and the validity of such assumptions can be assessed. Test piles are commonly instrumented with strain gages and/or inclinometers. P-y curves can be back computed from these data, and the curves obtained with the two methods should agree closely. Results from a field test on a concrete pile are presented and the critical factors for the analysis are discussed. One added difficulty with concrete is its nonlinear behavior particularly near structural failure. The curves obtained are also compared with those developed from the Dilatometer/Cone Pressuremeter Test and the Standard Penetration Test.