Small cell lung cancer (SCLC) is a frequently occurring, highly aggressive tumor with a generally poor clinical outcome. In order to approach the genetic mechanisms behind the tumor progression, a general screen for DNA copy number alterations was performed using comparative genomic hybridization (CGH). In the series of 23 cases analyzed, CGH alterations were frequently detected ranging from 9 to 22 abnormalities in the individual tumors. The most frequent losses were detected on chromosome arms 3p (23/23), 13q14-21 (23/23), 4p (20/23), 4q (20/23), and 2q22-24 (18/23), while gains preferentially involved chromosome arms 19p (18/23), 19q (17/23), 1p31-35 (15/23), 17q22-25 (11/23), and 5p14-15.3 (9/23). In addition, high level amplification at chromosome arms 1p32-33 and 2p22-24 were found in three and two cases, respectively. Candidate genes for these amplifications include the l-MYC (1p32) and n-MYC (2p24.1) oncogenes, which have been previously found to be overexpressed in SCLC. Taken together, the findings demonstrate a high level of chromosomal instability in SCLC, which is well in agreement with the highly malignant phenotype of this tumor type. Subchromosomal regions involved in gains and losses were delineated and the amplifications of 1p32-33 and 2p22-24 were demonstrated, thus providing starting points for the exact characterization of molecular events involved in SCLC tumor progression.