About 20 minutes into the episode, there was a scene in which Pikachu stops some vaccine missiles with its Thunderbolt attack, resulting in a huge explosion that flashed red and blue lights. Although there were similar parts in the episode with red and blue flashes, an anime technique called "paka paka" made this scene extremely intense, for these flashes were extremely bright strobe lights, with blinks at a rate of about 12 Hz for approximately 4 seconds in almost fullscreen, and then for 2 seconds outright fullscreen. At this point, viewers started to complain of blurred vision, headaches, dizziness and nausea. A few people even had seizures, blindness, convulsions and lost consciousness. Japan's Fire Defense Agency reported that a total of 685 viewers (310 boys, 375 girls) were taken to hospitals by ambulances. Although many victims recovered during the ambulance trip, more than 150 of them were admitted to hospitals. Two people remained hospitalized for over 3 weeks. Some other people had seizures when parts of the scene were rebroadcast during news reports on the seizures. Most of them were not diagnosed with photosensitive epilepsy. Later studies showed that 5-10.4% of Pokémon viewers had mild symptoms that did not need hospital treatment. 120,000 children reported mild symptoms of illness, however their symptoms more closely resembled mass hysteria than a grand mal seizure. A study following 103 patients over three years after the event found that most of them had no further seizures.
Another link: http://www.csicop.org/si/2001-05/pokemon.html