Usability is defined by the International Standards Organization as "the effectiveness, efficiency and satisfaction with which a certain user may achieve a specific objective in a particular environment" (ISO DIS 9241-11). This definition highlights the need for considering the specific destination of a certain technology and reflects the current trends in Human-Computer Interaction (HCI). In compliance with these remarks, the evaluation will be described here of a Virtual Environment (VE) for the treatment of male sexual dysfunctions; the common assumption will be avoided according to which a VE is a space separated from its "real" surroundings [6, 7] and the full environment where the Virtual session takes place will be addressed instead. After a description of the conceptual framework adopted, the paper will dwell on one method among those deployed for the evaluation, namely the analysis of "situated actions". Four aspects will be dealt with: (a) the interplay of various concurrent settings during the virtual session; (b) the users' comprehension of the symbols used in the VE; (c) the structure of the relationship between users and guide; (d) the breakdowns during the human-VE interaction. The goals and the intended meanings of the simulation as set by the designers became the main parameters for the evaluation.