Six human neuroblastomas were analyzed by Giemsa and fluorescence banding techniques to identify chromosomal aberrations. Two neuroblastomas were primary tumors from untreated children, and four were well lines established from human neuroblastomas. Five of the six tumors studied were diploid or near diploid; one was near tetraploid. A 1p- was found in three of the neuroblastomas examined. The 1p-was present in both primary tumors, and in one it was the only abnormality detected. This deletion was also found in the cells of an established line, in addition to other abnormalities. Giant markers of different origins were found in the four cell lines, and no double-minute chromosomes were found in the primaries or the cell lines studied. Thus, a 1p-deletion was the most consistent abnormality found in the six human neuroblastomas examined in this study. We attempt to correlate this finding with Knudson's hypothesis on the origin of childhood cancer. Additional studies of primary tumors should clarify whether this specific chromosomal abnormality is related to the the acquisition of malignant behavior in human neuroblastomas.