Summary 1. One hundred fifty-six patients with petit mal epilepsy were treated withTridione. One hundred four, or 62 per cent showed a definite improvement. Fifty-two either failed to respond favorably or manifested serious toxic effects requiring withdrawal of the drug. 2. Sixty-two patients, or 39 per cent, developed undesirable reactions while taking Tridione. It was necessary to discontinue the Tridione in thirty-eight of these patients. 3. Eleven of the sixty-six patients with petit mal epilepsy developed theirfirst grand mal seizure soon after starting Tridione. 4. In thirteen of the ninety patients with mixed type of seizures (grand mal and petit mal) the grand mal seizures became more frequent and more severe soon after starting Tridione.