Computer codes based on Monte Carlo calculations have been extensively developed for the computation of the efficiency in gamma-ray spectrometry. The errors in the specific parameters of the detector due to the lack of precise knowledge of its characteristics usually represent one of the most important sources of inaccuracy in this simulation technique. We present here the influence of several detector parameters on the efficiency for a typical coaxial n-type HpGe detector. Calculations of the full-energy peak efficiencies were performed by means of a Monte Carlo code in the range 122–1836 keV for several types of source configuration: point source, cellulose filter, and two different cylindrical boxes containing a solid matrix of SiO2. The detector parameters varied were the crystal diameter, crystal height, diameter of the internal core, and the position of the crystal with respect to the beryllium window. The study shows that significant deviations in the efficiency, depending on the source geometry and the photon energy, can be produced by varying only slightly some of the detector parameters.