1. INTRODUCTION. Training in the diagnosis of arrhythmias is an important part of the curriculum for medical students, postgraduates, and paramedical staff. Although several CAI for arrhythmia have been developed [1-3], we could not get CAI software for arrhythmia for the MS-Windows environment. In this report, we present a newly-developed computer-assisted reference system for arrhythmia that functions in the Windows environment. 2. DESCRIPTION OF THE SYSTEM. The system consists of a program and two data files. An MS-Windows program (ECG9405.EXE, 180kB) was compiled using Borland's C++ v.3.1. A binary file (ECPAT.BAS 33kB) includes data of normal and abnormal wave segments of ECG: P wave, PQ interval segment, and QRs complex with/without T wave. A mother file (ECG9405.sys, 57kB) includes 85 data sets to generate ECG waveforms of arrhythmia. Each data set contains a sequence of wave form numbers, the text for questions and answers, and the commands strings. There are five major commands: 1) to create a new window as "wave window"; 2) to make electrocardiogram data; 3) to plot the data on the window; 4) to create a "dialog box" for questions and explanations; and 5) to check the answers. he program gets a data set from the data according to the user's choice. The program then interprets the data set and executes the commands. The wave segment data are plotted in a "wave window" at every 10 milliseconds; this is controlled by the MS-Windows' timer. The timer interval can be changed by selecting the speed button. The ECG waveforms are displayed on a window just like an ordinary ECG monitor with beat sound. Many windows can be created by the user and many ECG waves simultaneously plotted on CRT. 3. USAGE OF THE SYSTEM. The "main window" has a menu that has three items corresponding to the training course: BASIC, TRY, and TEST. Thirty-five types of arrythmias are listed in the "list box" of the windows in BASIC course e.g., sinus arrhythmia, atrial flutter, atrial premature contraction, ventricular extrasystole, ventricular flutter, etc. If the user selects one of them on the list by double clicking, some textual explanations of the wave are described in a dialog box. Ten multiple choice questions are displayed in the dialog box in course of learning TRY and TEST; the answers to these are requested. In the TEST course, the system offers random access to each arrhythmia. he user can send the pictorial ECG data in the window to other graphics programs through a clip board. 4. DISCUSSION. It was successfully used in a lecture of electrocardiogram for medical students. They seem to be interested in this system because of its simple usage and the dynamic drawing of ECG waves on CRT. Multiple computer-based medical resources can be run on MS-Windows. The system is able to run simultaneously with other programs, such as an electronic reference system [4]. The system may be obtained from the authors upon request.