Four families with X-chromosome linked retinitis pigmentosa (14 patients, 14 carriers and 11 normal relatives) were investigated with full-field ERG with computer averaging and an electronic narrow bandpass filter. Reliable ERGs were obtained with 30 Hz flickering white light from 13 out of 14 patients with the disease, all severely affected. Further, B-wave implicit times were found to be more informative than B-wave amplitudes for identifying carriers of the disease. Full-field ERG appears to be a valuable tool for studying X-chromosome linked retinitis pigmentosa families, when necessary supported with computer averaging and a narrow bandpass filter.