Recent family-based studies have revealed evidence for linkage of chromosomal region 12q to both asthma and high total serum immunoglobulin E (IgE) levels. Among the candidate genes in this region for asthma is neuronal nitric oxide synthase (NOS1). We sought a genetic association between a polymorphism in the NOS1 gene and the diagnosis of asthma, using a case-control design. Frequencies for allele 17 and 18 of a CA repeat in exon 29 of the NOS1 gene were significantly different between 490 asthmatic and 350 control subjects. Allele 17 was more common in the asthmatics (0.83 vs 0.76, or 1.49 [95% CI 1.17-1.90], P = 0.013) while. allele 18 was less common in the asthmatics (0.06 vs 0.12, or 0.49 [95% CI 0.34-0.69], P = 0.0004). To confirm these results we genotyped an additional 1131 control subjects and found the frequencies of alleles 17 and 18 to be virtually identical to those ascertained in our original control subjects. Total serum IgE was not associated with any allele of the polymorphism. These findings provide support, from case-control association analysis, for NOS1 as a candidate gene for asthma.