The three-dimensional magnetic field structure of 137 solar active regions is studied by comparing the observed and computed chromospheric magnetograms. The model chromospheric field is obtained by extrapolating the observed photospheric field into the chromosphere with a potential (current-free) magnetic-field model in Cartesian geometry. The best correlation between the observed and the model chromospheric magnetograms is found at the height of 800 km, which also corresponds to the height of the line formation for Ca II 854.2 nm. In the weak field range, within ±300 G, most of the observed field is close to the potential field. However, departures of about 50 G are observed in a few active regions. For field values greater than ±500 G, the observed field does not always match the model. Whereas a part of this could be due to the magnetogram calibration, it might also originate from the "nonpotentiality" of the chromospheric field. In the case of long-lived active regions, which make multiple disk passages, the strong-field nonpotentiality is observed during their initial phase, and converges to a potential field configuration later.