The second Caucasian xeroderma pigmentosum patient (XP42RO) belonging to complementation group F (XP-F) is described. Mild ocular photophobia was present from childhood, and acute skin reactions occurred upon exposure to sunlight. Basal and squamous cell carcinomas developed after his twenty-seventh year. In his late forties progressive neurologic symptoms emerged, which included intellectual decline, mild chorea and ataxia, and marked cerebral and cerebellar atrophy. Such neurologic abnormalities are very unusual in XP-F. Similar symptoms have been described in only one of 17 other XP-F individuals. His approximately 5-fold reduced activity of nucleotide excision repair in cultured cells, combined with moderately affected cell survival and DNA replication after UV exposure, are typical of XP-F. The recent cloning of the XPF gene allowed a molecular genetic analysis of this unusual patient. XP42RO, representing the second case studied in this respect, turned out to be homozygous for a point mutation in the XPF gene, causing an R788-->W substitution in the encoded protein. Surprisingly, this mutation had also been found in one allele of the other unrelated Caucasian XP-F case. The amount of mutated XPF protein is strongly reduced in cells from XP42RO, presumably due to a conformational change. Biochemical, genetic, and clinical data all indicate the presence of considerable residual repair activity, strongly suggesting that the R788W mutation is leaky.