In childhood malignancies such as retinoblastoma and Wilms tumour, of which both familial and sporadic forms exist, recessive mutations of presumed differentiation genes have been implicated in tumorigenesis^1,2. A proportion of cases appear with microscopically visible chromosome deletions which indicate the regions where the genes concerned are located. Mutation or loss of one allele causes a cancer predisposition. For tumour development functional loss of the remaining normal allele is also required. In cancers with both familial and sporadic forms, molecular-genetic studies have shown that deletion is often one of the mutational events^2–5.Although familial and sporadic forms have never been distinguished in lung cancer, deletions of the short arm of chromosome 3 have been described for small cell lung cancer (SCLC) (refs 6, 7), but their general occurrence in SCLC has been disputed^8–10. Using a molecular-genetic approach, we here present evidence for a consistent deletion at the chromosomal region 3p21, not only in SCLC¹¹, but in all major types of lung cancer.