A specific, acquired chromosomal abnormality (deletion 3p) has been found in at least one chromosome 3 in 100 percent of the metaphases in 12 of 12 cell lines cultured from human small-cell lung cancer tissue and in 2-day tumor culture specimens from three patients. Analysis of the shortest region of overlap shows the deletion to be 3p(14-23). This specific change was not seen in five of five lung cancer cell lines other than small-cell lung cancer or in two lymphoblastoid lines cultured from cells of small-cell lung cancer patients whose tumors had the 3p deletion.