The results of three full-scale lateral load tests of cast-in-drilled-hole RC piles are evaluated to provide insights into the soil-pile interaction behavior as represented by p-y curves (p 5 soil reaction/length; y 5 lateral pile deflection) spanning from elastic conditions to mobilization of the soil capacity. The test sequence enables evaluation of the diameter effects on the p-y behavior as a result of testing two otherwise similar specimens, but with different diameters (0.6 and 1.8 m), and evaluation of the head-fixity effects as a result of testing similar specimens (with a 0.6 m diameter) under flagpole and fixed-head conditions. A constrained exhaustive search procedure that facilitates data interpretation even in the presence of severe shaft nonlinearity is introduced, which was needed because the testing was performed to full structural failure conditions. The p-y curves obtained with the present test data differ from the predictions of a standard (American Petroleum Institute) model, indicating stronger soil reactions for the fixed-head and 1.8-m-diameter flagpole specimens and weaker and softer backbone curves for the 0.6-m-diameter flagpole test.