DNA was prepared from tumour and normal tissue from 48 patients representing all common histological types of nonsmall-cell lung cancer. Using eight DNA probes, which detect nine restriction enzyme fragment length polymorphisms (RFLP) on chromosome 3, we established that among the 44 informative patients 32 had lost alleles on the short arm of one of their copies of chromosome 3. Of these 32, at least 13 had also lost alleles on the long arm of chromosome 3, suggesting that the whole chromosome might be lost. For one patient, cytogenetic analysis indicated that the mechanism of allelic loss was reciprocal translocation followed by chromosomal loss of one of the reciprocal products. Two patients with allelic loss distal to the D3S3 locus (which maps to 3p13–14) retained heterozygosity at that locus. These results indicate that loss of alleles on the short arm of chromosome 3 is a common event in lung tumours of the nonsmall-cell type, that this loss occurs by a variety of chromosomal mechanisms, and that the minimally deleted region is 3p13–14→3pter.