We consider the Banach function spaces, called fully measurable grand Lebesgue spaces, associated with the function norm ( ) = ess sup ∈ ( ) ( ) ( ), where ( ) denotes the norm of the Lebesgue space of exponent ( ), and (⋅) and (⋅) are measurable functions over a measure space ( , ]), ( ) ∈ [1, ∞], and ( ) ∈ (0, 1] almost everywhere. We prove that every such space can be expressed equivalently replacing (⋅) and (⋅) with functions defined everywhere on the interval (0, 1), decreasing and increasing, respectively (hence the full measurability assumption in the definition does not give an effective generalization with respect to the pointwise monotone assumption and the essential supremum can be replaced with the simple supremum). In particular, we show that, in the case of bounded (⋅), the class of fully measurable Lebesgue spaces coincides with the class of generalized grand Lebesgue spaces introduced by Capone, Formica, and Giova.Let (Ω, ) be a finite measure space, and let M(Ω) be the set of all measurable functions on Ω with values in [−∞, +∞], M + (Ω) be the subset of the nonnegative functions, and M 0 (Ω) be the subset of the real valued functions.Let ( , ]) be a measure space, and define M( ) similarly as above.whereThen [⋅], (⋅) is a Banach function norm, and the associated Banach function spaceis called fully measurable grand Lebesgue space. This definition is one of the most abstract generalizations of the original grand Lebesgue spaces originated in the paper by Iwaniec and Sbordone [2]. Given (⋅) ∈ M( ) as above, let us setso that 1 ≤ − ≤ + ≤ ∞. In the following, we exclude the noninteresting case of constant (⋅), that is, the case − = + š 0 , because in such case we know (see [1]) that [⋅], (⋅) (Ω) = 0 (Ω).