In this work we examine the relation between the sensitivity (well posedness) and relaxability for nonlinear distributed parameter systems. We introduce the notion of “strong calmness” which describes the dependence of the value of the problem on the state constraints, and we show that it is equivalent to “relaxability”. We also present an alternative control free description of the relaxed problem. An example of a nonlinear parabolic optimal control problem of Lagrange type is worked out in detail.