  Let $H_n$ be the $n$-th harmonic number and let $v_n$ be its denominator. It
is well known that $v_n$ is even for every integer $n\ge 2$. In this paper, we
study the properties of $v_n$. One of our results is: the set of positive
integers $n$ such that $v_n$ is divisible by the least common multiple of $1,
2, \cdots, \lfloor {n^{1/4}}\rfloor $ has density one. In particular, for any
positive integer $m$, the set of positive integers $n$ such that $v_n$ is
divisible by $m$ has density one.
