The theory of unpleasant symptoms is a middle-range theory proposed by Lenz and her colleagues (1997). Analysis and evaluation of this theory was performed using Fawcett (1999) and Fawcett and Downs's (1992) guidelines. Results of the theory analysis and evaluation suggest that the theory of unpleasant symptoms has theoretical and social significance and parsimony. However, a lack of internal consistency was evident. For empirical adequacy of the theory, it is recommended that research be conducted examining the complexities of the interaction effects, reciprocal relationships, and medication effects among physiological, psychologic, and situational factors, symptoms, and performance. The knowledge derived from the research findings should be used in practice for patients experiencing symptoms.