A family with autosomal dominant retinitis pigmentosa showing reduced penetrance is described. Reduced penetrance is established in patients who transmitted the gene defect to their offspring but are themselves clinically normal and beyond the age of risk for the disease. The electroretinograms (ERG) demonstrate rod responses reduced in amplitude and delayed in implicit time at an early stage similar to those seen in dominant retinitis pigmentosa with complete penetrance. Cone responses are also delayed in implicit time even when response amplitudes are normal. This cone ERG abnormality has not been found in dominant retinitis pigmentosa with complete penetrance.