Twenty-two of 23 obligate female carriers in nine families with known X-chromosome-linked retinitis pigmentosa were detected on the basis of abnormal full-field electroretinograms (ERGs). Only 14 of these carriers had fundus findings characteristic of the carrier state. Electroretinograms of carriers were either reduced in amplitude to white light under dark-adapted conditions or delayed in cone b-wave implicit time, or both. Daughters of obligate carriers had either normal ERGs or abnormal ERGs similar to those recorded from obligate carriers. Abnormal ERGs of carriers of X-chromosome-linked retinitis pigmentosa contrasted with the normal ERGs recorded from female carriers of autosomal recessive disease. These data support the idea that ERG testing of female relatives of males with retinitis pigmentosa can help to establish for a given family whether the mode of inheritance is X-chromosome-linked or autosomal recessive.