Which Pokémon anime episode was withdrawn after causing reports of seizures among viewers?

The Pokémon anime episode withdrawn after reports of seizures among viewers was Dennō Senshi Porygon.

The episode aired in Japan on December 16, 1997. A sequence near the end used rapidly alternating red and blue flashes during an explosion. Thousands of viewers reported physical symptoms, including seizures, headaches, dizziness, and nausea; the incident became known internationally as the Pokémon shock.

The episode was removed from circulation and has not received a standard television rebroadcast. The event prompted investigations and led Japanese broadcasters and animation producers to adopt stronger precautions for flashing imagery. The Pokémon television series was also temporarily taken off the air while the consequences were assessed.

Although Porygon appears in the episode, the flashing sequence was performed by Pikachu's attack rather than by Porygon itself. This is why the episode is often incorrectly called the Pikachu episode or blamed solely on Porygon. The title is commonly rendered as Dennō Senshi Porygon, meaning Computer Warrior Porygon.