The investigation of a case of disputed paternity revealed indirect exclusion of the alleged father in the haptoglobin system and in the DNA single-locus system D16S309/Hinf I (MS205). The paternity index for the non-exclusion systems was > 10(6). Since both exclusion systems (HP and MS205) are located on chromosome 16, we investigated 10 microsatellite loci covering this chromosome with 10-20 cM resolution. Analysis of the child's chromosome showed only alleles of maternal origin and lack of inheritance of paternal alleles for five informative loci. The markers close to the centromere of chromosome 16 were heterozygous, whereas distal loci were either heterozygous or homozygous for maternal alleles. This is consistent with a maternal meiosis I nondisjunction of chromosome 16 leading to maternal uniparental heterodisomy. This case emphasizes that the opinion of non-paternity should be based on the absence of paternal alleles at genetic systems located on at least two different chromosomes.