Atopy is a common familial state underlying allergic asthma and rhinitis. Lately, we have assigned a gene for atopy to chromosome 11q by linkage to the marker D11S97. Since previous studies have suggested that the risk of atopy is higher for children of atopic mothers than for those of atopic fathers, we sought differences between maternal and paternal patterns of transmission at the 11q13 locus among pairs of siblings in families affected by atopy. When we defined atopy as the presence of a positive skinprick test (greater than or equal to 2 mm) to any of a panel of common allergens, a higher than normal concentration of total serum IgE, or a positive radioallergosorbent test for a specific IgE, we found that 125 (62%) of the sibling-pairs affected by atopy shared the maternal 11q13 allele and 78 (38%) did not. This distribution differs significantly from the expected 50/50 distribution (p = 0.001). Of paternally derived alleles, 83 (46%) were shared and 96 (54%) were not (not significantly different from 50/50). The result was similar whatever definition of atopy was used and with other genetic markers on 11q. These findings show that transmission of atopy at the chromosome 11q locus is detectable only through the maternal line. The pattern of inheritance is consistent either with paternal genomic imprinting or with maternal modification of developing immune responses.