  It has long been conjectured that the classical Lorenz attractor supports a
unique measure of maximal entropy. In this article, we give a positive answer
to this conjecture and its higher-dimensional counterpart by considering the
uniqueness of equilibrium states for H\"older continuous functions on a
sectional-hyperbolic attractor $\Lambda$. We prove that in a $C^1$-open and
dense family of vector fields (including the classical Lorenz attractor), if
the point masses at singularities are not equilibrium states, then there exists
a unique equilibrium state supported on $\Lambda$. In particular, there exists
a unique measure of maximal entropy for the flow $X|_\Lambda$.
