We have studied the molecular genetics of 27 Brazilian families with ataxia telangiectasia (AT). Five founder effect haplotypes accounted for 55.5 per cent of the families. AT is an autosomal recessive disorder of childhoodonset characterized by progressive cerebellar ataxia, ocular apraxia, telangiectasia, immunodeficiency, radiation sensitivity, chromosomal instability, and predispositionto cancer. The ATM gene spans more than 150 kb on chromosome region 11q23.1 and encodes a product of 3,056 amino acids. The ATM protein is a member of the phosphatidylinositol 3-kinase (PI-3K) family of proteins and is involved in cell cycle control and DNA repair pathways. DNA was isolatedfrom lymphoblastoid cell lines and haplotyped using four STR markers (D11S1818, NS22, D11S2179, D11S1819) within and flankingthe ATM gene; all allele sizes were standardized in advance. In addition to the STR haplotypes,SNP haplotypes were determined using 10 critical polymorphisms. The entire gene was screened sequentially by protein truncation testing (PTT), single strand conformation polymorphism (SSCP), and then denaturing high performance liquid chromatography (dHPLC) to identify the disease-causing mutations. Of the expected 54 mutations, 50 were identified. All mutations but one, led to a truncated or null form of the ATM protein (nonsense, splice site, or frameshift). Five families (18.5 per cent) carried a deletion of 3450nt (from IVS28 to Ex31), making this one of the two most common Brazilian mutations. Mutations were located throughout the entire gene, with no clustering or hotspots. Standardized STR haplotype analysis greatly enhanced the efficiency of mutation screening.