Of three patients with familial hypobetalipoproteinemia, a 42-yeear-old white woman, who was homozygous for this autosomal dominantly inherited disease, had no detectable serum betalipoprotein and had a marked retinal pigmentary degeneration characterized by ring scotomas by Goldmann perimetry, extinguished electroretinographic responses, delayed responses and elevated thresholds during dark adaptometry, and abnoramal cone thresholds. A 4-year-old daughter and a 28-year-old niece of the first patient, who wer heterozygous, had reduced but detectable levels of serum betalipoprotein and no significant retinal pigmentary degeneration. Unlike patients with autosomal recessively inherited abetalipoproteinemia (the Bassen-Kornzweig syndrome), none of our patients had significant neurologic of cardiac defects. Although the level of serum betalipoprotein might be correlated with retinal pigmentary degeneration in familial hypobetalipoproteinemia and abetalipoproteinemia, it appears that neurologic and cardiac defects are dependent on other factors.