Many patients with Parkinson's disease (PD) have clinically significant anxiety, depression, fatigue, sleep disturbance, or sensory symptoms. The comorbidity of these nonmotor symptoms and their relationship to PD severity has not been extensively evaluated. Ninety- nine nondemented PD patients were evaluated with the following battery of tests: Beck Anxiety Inventory (BAI), Beck Depression Inventory (BDI), Fatigue Severity Scale (FSS), Pittsburgh Sleep Quality Index (PSQI), a sensory symptom questionnaire, Unified Parkinson's Disease Rating Scale (UPDRS), Hoehn & Yahr (H/Y) Stage, and the Schwab & England ADL scale (S/E). The comorbidity of the nonmotor symptoms and their relationship to PD severity was analyzed. Thirty-six percent of the study population had depression (BDI ≥10), 33% had anxiety (BAI ≥10), 40% had fatigue (FSS > 4), 47% had sleep disturbance (PSQI > 5), and 63% reported sensory symptoms. Only 12% of the sample had no nonmotor symptoms. Fifty-nine percent of the patients had two or more nonmotor symptoms, and nearly 25% had four or more. Increased comorbidity was associated with greater PD severity (P < 001). This study reveals that the nonmotor symptoms of PD frequently occur together in the same patients. Increased comorbidity of the five nonmotor symptoms was associated with greater PD severity. These results suggest that recognition of these diverse nonmotor symptoms may be enhanced by looking for others when one nonmotor symptom has been identified. © 2001 Movement Disorder Society.