Ocular findings are presented from 17 unrelated patients with a form of autosomal dominant retinitis pigmentosa and the same cytosine-to-adenine transversion in codon 23 of the rhodopsin gene corresponding to a substitution of histidine for proline in the 23rd amino acid of rhodopsin (designated rhodopsin, Pro-23-His). On average, these patients (mean age, 37 years) had significantly better visual acuity and larger electroretinographic amplitudes than 131 unrelated patients (mean age, 32 years) with autosomal dominant retinitis pigmentosa without this mutation. However, these 17 patients from separate families, as well as 12 relatives with the mutation from four of these families, showed interfamilial and intrafamilial variability with respect to severity of their ocular disease, suggesting that some factor(s) other than this gene defect itself is involved in the expression of their condition. This form of retinitis pigmentosa can now be detected by testing leukocyte DNA from peripheral blood. Some mechanisms by which this mutation in the rhodopsin gene could lead to rod photoreceptor cell death are suggested.