It can be difficult to detect upper motor neuron (UMN) involvement in early amyotrophic lateral sclerosis (ALS). The present study tested the usefulness of mirror movements (MMs), i.e., contralateral coactivation of hand muscles, as a UMN sign in ALS. Thirty-seven patients with possible, probable or definite ALS and 19 patients with suspected ALS without clinical signs of UMN involvement, 21 disease controls and 15 healthy volunteers were included. MMs were studied clinically, electromyographically and by transcranial magnetic stimulation (TMS), looking for ipsilateral motor evoked potentials (IMEPs). MMs were observed clinically in 15 ALS patients (27%) and electromyographically in 28 (50%). IMEPs in the abductor pollicis brevis muscle following TMS were recorded in 34 (61%) of all ALS patients and in nine (47%) out of 19 patients with suspected ALS, but not in healthy controls. Central motor conduction times were prolonged in 15 ALS patients (27%). It is concluded that MMs, especially if studied by EMG and TMS (IMEPs) can essentially improve detection of UMN involvement, especially in early stages of ALS.