The ballistocardiograms (BCGs) of the cases with auricular fibrillation are generally complicated in their waves. Therefore, it is not easy to analyse the BCGs of such patients by routine method. Harmonic analysis was applied in order to interprete such BCG accurately. The examines consisted of 15 cases of auricular fibrillation with mitral valvular disease and 30 cases of auricular fibrillation with hypertensive heart diseases and 10 healthy persons, totaling 55 cases. The longitudinal BCGs were recorded during rest breathing for one minute on a magnetic tape by using von Wittern-type sublow-frequency table and a Sony data recorder (PWA-23, MA-33). The recorded magnetic tape was rerecorded on a magnetic tape of a Sony correlater (CCA-32 type). Autocorrelograms of BCGs were obtained by setting the correlater with the rerecorded magnetic tape. All the autocorrelograms of the patients of auricular fibrillation showed damping patterns, and they were classified into three types. Type 1 consists mainly of aperiodic component with a little random component. Type 2 consists of aperiodic component and of random component. Type 3 consists only of random component. Among the autocorrelograms of BCGs of 45 cases with auricular fibrillation, 29 cases (44%) belonged to Type 1, 15 cases (33%) to Type 2, and 10 cases (22%) to type 3. The power density spectra of BCGs were obtained by the Fourier transformation of the autocorrelation function. The power density spectrum of BCG in the auricular fibrillation. either with mitral valvular diseases or with hypertensive heart diseases, damped slowly, and then converged on the zero line about 10 cps. But it showed an abnormal fluctuation over the frequency zone from 3 cps to 6 cps in all the cases of the auricular fibrillation with mitral valvular diseases. The curves of the power density spectra of BCGs showed sinusoidal wave patterns of small amplitude in all the cases of auricular fibrillation with hypertensive heart diseases. This pattern differed distinctly from that of auricular fibrillation with mitral valvular diseases. In healthy persons, the curves of the power density spectra of BCGs showed regular and damping sinusoidal wave patterns.