Application of chaos analysis to BIST 100 return series Long-term memory effect and linearity test of BIST 100 returns Short-term return forecast of the BIST 100 index Figure A. Process steps applied to BIST 100 returns Purpose: The realization of short-term forecasts of BIST 100 returns provides company managers with the advantage of keeping company values high, while providing investors with opportunities to reduce transaction costs and provide advantages. In this paper, it is aimed to investigate whether the BIST 100 return series has a chaotic structure and to predict successfully in the short term. Theory and Methods: To analyze the chaos of the BIST 100 return series, reconstruction of phase space, correlation dimension and Lyapunov exponential methods were used. However, linearity of this series was examined by BDS (Brock, Dechert and Scheinkman) test. In the next step, the Hurst exponential coefficient, which shows whether this series has a long-term memory effect, was determined by the transformed width method. Then, Artificial Neural Networks (ANN) and Adaptive Network Based Fuzzy Inference System (ANFIS) methods were used in the estimation of the BIST 100 index. BIST 100 returns were obtained by applying logarithmic transformations to the outputs of the model that was successful in index estimation. Results: As a result of the chaos analysis, it was found that the BIST 100 return series had a chaotic structure. Then, it was observed that this series was not linear with BDS test. However, Hurst exponential coefficient was examined and it was concluded that this series had long term memory effect. Then, as a result of estimating this series with the most successful model, the mean absolute percentage error percentage (MAPE) value was found between 10%-20%. This shows that the BIST 100 return estimate is consistent. Conclusion: The ANFIS method shows a successful result in the short-term estimation of BIST 100 returns with chaotic properties.