Uniparental disomy (UPD)-the inheritance of two homologous chromosomes from a single parent-may be unmasked in humans by the unexpected appearance of developmental abnormalities, genetic disorders resulting from genomic imprinting, or recessive traits. Here we report a female patient with familial chylomicronemia resulting from complete lipoprotein-lipase (LPL) deficiency due to homozygosity for a frameshift mutation in exon 2 of the LPL gene. She was the normal term product of an unremarkable pregnancy and had shown normal development until her current age of 5.5 years. The father (age 33 years) and the mother (age 24 years) were unrelated and healthy, with no family history of stillbirths or malformations. The father was a heterozygous carrier of the mutation, whereas no mutation in the LPL gene was detected in the mother. Southern blotting did not reveal any LPL gene rearrangement in the proband or her parents. The proband was homozygous for 17 informative markers spanning both arms of chromosome 8 and specifically for the haplotype containing the paternally derived LPL gene. This shows that homozygosity for the defective mutation in the LPL gene resulted from a complete paternal isodisomy for chromosome 8. This is the first report of UPD for chromosome 8 unmasked by LPL deficiency and suggests that normal development can occur with two paternally derived copies of human chromosome 8.