This paper evaluates the laboratory and environmental factors that induce seizures in photosensitive epileptic patients. The aims are (i) to identify those factors that trigger seizures so that the patients could take preventive measures, (ii) to advise the patients on appropriate preventive measures that could be taken to avoid spontaneous seizures. Thirty-six photosensitive epileptic patients were studied. The electroencephalographic laboratory seizure-inducing factors identified were intermittent photic stimulation (IPS), flickering lights from both the TV and the visual display unit (VDU) at various flashes per second, grid patterns (vertical, horizontal lines, squares, black and white lines) of various cycles per degree, and blank screens. In the environmental factor investigation, 10 out of 36 patients had series of seizures induced by narrow stripes of black and white, or white and red striped coloured dresses especially, when stepping outdoors on sunny days. Other environmental factors identified include moving escalators; 'moving' trees, bushes, and houses when travelling in a vehicle, including trains; faceted patterns in the bathroom windows with light passing through; swirling light patterns especially on displayed objects of black and white colours; telegraphic post, gantry post, and luminance variance when stepping outdoors from the dark in bright sunlight. The effective preventive measures were: (i) patients should occlude one eye when travelling in a vehicle or using computers or when stepping outdoors on a sunny day, or when there are various patterns before them, (ii) patients should sit at least 3 metres away from the TV when watching a programme, (iii) they should avoid any object that transmits luminance variance; if this is not possible, they should occlude one eye.