The analysis of oncogene expression may provide insights into the pathogenesis of small cell lung cancer (SCLC) and may help to predict clinical behavior. The expression of 8 oncogenes (c-myc, N-myc, L-myc, Ha-ras, Ki-ras, N-ras, erbB-2, v-sis) was evaluated in small cell lung cancer (SCLC) xenografts of tumor samples, recentlly transplanted, taken from 17 different patients. Eight of these 17 SCLC lines expressed the L-myc oncogene and 2 SCLC lines expressed the c-myc oncogene. One SCLC line (SCLC-63M) simultaneously expressed the L-myc and c-myc oncogenes. All SCLC lines examined had almost similar high RNA levels of the Ki-ras oncogene, while the expression of Ha- and N-ras oncogenes was not always observed. The N-myc and v-sis oncogenes were expressed in only one tumor and at a very weak level, and no transcript of the erbB-2 oncogene was observed in any of our 17 SCLC lines. These results indicate that oncogene expression in SCLC lines is heterogeneous, with the exception of the Ki-ras oncogene which is constantly overexpressed.