The banding automatic system (BAS) we describe in this paper is a computer-assisted karyotyping system suitable for the analysis of both normal and abnormal karyotypes in banded chromosome specimens. In the BAS system recognition of the various features and constituents of the chromosomes is performed by an operator, whereas chromosome identification and classification are entrusted to a computer. The connection between operator and computer is the CYGENl program, which is tailored to distinguish chromosome morphology on the basis of the operator input band patterns alone. The karyotyping operation is therefore significantly speeded up: the operator simply has to read through a certain number of banded metaphases, encoding band-pattern data to the computer. The machine signals when the operation has gone through the whole karyotype or, for instance, when chromosomes are missing or present in extra number above the significance level from the analyzed metaphases. Chromosome identification and classification are then carried out by the computer, which performs as a real-time recognizing key. Moreover, the data being accumulated in the computer will provide a file of cytogenetic records accessible to the operator, as well as to the computer, for further diagnostic operations. The efficacy of the BAS system is demonstrated for the diagnosis of various structural aberrations, including a complex translocation. The processing of 20 metaphases by the method described takes 40–45 min, and a cytogenetic technician can make effective use of the system after 25–30 h of intensive train