Eleven patients with X-linked and 9 patients with autosomal recessive achromatopsia were examined with full-field electroretinograms. In the standard full-field ERG's, normal rod responses were obtained, but the amplitude of the cone b-waves was not detectable. With computer averaging and narrow bandpass filtering, residual cone b-wave responses could be detected in 10 of the 20 patients. The residual cone b-wave amplitudes were markedly different in the 3 families with X-linked achromatopsia. In two of them, residual cone b-wave responses were seen in all patients examined. In contrast, such responses were seen only in 2 of 7 patients in the third family. There were also differences in other clinical observations (mainly in the visual acuity and refractive error) and we therefore suggest that there are at least two forms of X-linked achromatopsia. The ratios of the cone response amplitudes to 30 Hz flickering orange and blue-green light suggested that the defect in the X-linked achromatopsia patients was of the protanope type, whereas in the autosomal patients, both the protanope and the deutanope type was seen. In conclusion, measurements of the residual cone b-wave amplitude responses are of diagnostic and may possibly be of prognostic value when examining children and other members of families with achromatopsia.