Introduction:
Patients with multiple sclerosis (MS) experience fatigue as a subjective symptom with inter-individual variability. Fatigue is present at any time during the course of the disease, with varying durations over a period of time.

Aim of the research:
To determine the conceptual definition of fatigue in adult patients with MS and to describe fatigue over a given time period.

Material and methods:
147 patients with MS (Group A) and 158 controls (Group B) participated in the quantitative prospective cross-sectional study. The questionnaire contained items focused on conceptual definition of fatigue, its duration during the day and its frequency during the week. The c2 test of independence and the Student’s t-test for two independent samples were used for statistical analysis.

Results:
Group A significantly more often than Group B perceived fatigue as muscle weakness, physical tiredness, whole body exhaustion, lack or absence of physical and mental strength, greater effort when performing activities, tired/heavy legs, increased tiredness, memory difficulties, problems with concentration, impatience, despondency, and apathy. Group A reported significantly longer duration of fatigue (4.82 hours) than Group B (2.49 hours) in 24 hours. The incidence of fatigue in 7 days was significantly more frequent in Group A (5.26 days) than in Group B (3.01 days).

Conclusions:
Understanding the conceptual definition and duration of fatigue leads to an effective management of the disease symptoms.