Mutations in the lipoprotein lipase (LPL) gene are the most important cause of familial chylomicronemia with over 70 mutations being recorded to date. Thus far de novo mutations have not been described. Here we report on the molecular analysis of the family of a patient previously reported to be LPL deficient on the basis of compound heterozygosity for the Arg243His and Ile225Thr mutations, the latter being the first and only mutation identified in the loop region of LPL. Both parents of the propositus were screened for the presence of these two mutations to confirm their status as obligate heterozygotes and to determine the mutation allocation. Although paternal inheritance of the Arg243His allele could be established, maternal DNA did not show carrier status for the Ile225Thr substitution. An examination of maternity, using LPL restriction fragment length polymorphisms four polymorphic CA repeats and ApoE genotypes, was consistent with correct biological parentage for the propositus. Therefore, we conclude that the Ile225Thr mutation constitutes a de novo event, the first to be reported in the LPL gene.