We consider lattice coding over a Gaussian wiretap channel, where an eavesdropper listens to the transmissions between a transmitter and a legitimate receiver. In [1], a new lattice invariant called the secrecy gain was introduced as a code design criterion for wiretap lattice codes, shown to characterize the confusion that a chosen lattice code can cause at the eavesdropper: the higher the secrecy gain of the lattice, the more confusion. In this paper, a formula for the secrecy gain of unimodular lattices is derived. Secrecy gains of extremal odd unimodular lattices as well as unimodular lattices in dimension 16 are computed and compared. Finally, best wiretap lattice codes coming from unimodular lattices in dimension n, 8 ≤ n ≤ 16 are classified.