Ocular findings are presented in 17 unrelated patients with a form of autosomal dominant retinitis pigmentosa and the same C to A transversion in codon 23 of the rhodopsin gene. These patients (mean age, 36.6 years) had, on average, significantly better visual acuity and larger ERG amplitudes than 131 unrelated patients (mean age, 32.1 years) with autosomal dominant retinitis pigmentosa without this mutation. These 17 patients from separate families as well as 11 relatives with the mutation from 4 of these families showed interfamilial and intrafamilial variability with respect to severity of their ocular disease. This clinical heterogeneity among patients with the same mutation, with older patients sometimes showing less loss of visual function and less intraretinal bone spicule pigment than younger patients, suggests that some factor other than the gene defect itself is involved in the expression of this condition. This form of retinitis pigmentosa can now be detected by testing leukocyte DNA from peripheral blood. Patients so identified should have an ocular examination to determine the extent of their disease in view of the clinical heterogeneity that exists among patients with this mutation. Some mechanisms by which this mutation in the rhodopsin gene could lead to photoreceptor cell death are discussed. Opportunities for future clinical and laboratory research in search of possible treatments are considered.