  Inner ideals of simple locally finite dimensional Lie algebras over an
algebraically closed field of characteristic 0 are described. In particular, it
is shown that a simple locally finite dimensional Lie algebra has a non-zero
proper inner ideal if and only if it is of diagonal type. Regular inner ideals
of diagonal type Lie algebras are characterized in terms of left and right
ideals of the enveloping algebra. Regular inner ideals of finitary simple Lie
algebras are described.
