Background/Aim: Comorbid depression often occurs in chronic renal failure patients on hemodialysis (HD). Reduced serotonin (5-HT) function is implicated in the pathophysiology of depression. Methods: Comorbid depression and different clusters of depressive symptoms were assessed in 79 male HD patients and 35 male depressed psychiatric patients. Platelet 5-HT concentration (a peripheral model for the central serotonergic neurons) was determined in all patients and 80 male healthy controls. Results: Comorbid depression occurred in 50 out of 79 HD patients. Depressed psychiatric patients and depressed HD patients had higher scores of anxiety, retardation, and cognitive symptoms than nondepressed HD patients. Platelet 5-HT concentration was lower in depressed or nondepressed HD patients than in healthy controls, or in depressed patients. Higher platelet 5-HT content was found in depressed psychiatric patients with depressive clusters than in all other patients. Among HD patients, anxious HD patients had a higher platelet 5-HT concentration than HD patients without anxiety symptoms. Conclusions: Comorbid depression occurred in 63% of HD patients. Dialyzed patients had decreased platelet 5-HT concentration, regardless of the occurrence of comorbid depression. Higher platelet 5-HT concentration was related to anxiety symptoms in HD patients. Our data suggest that platelet 5-HT concentration might be a suitable marker for anxiety symptoms in dialyzed patients.