The main aims of psychiatric occupational therapy are to improve daily activity, to enhance communication with others and to reinforce social adaptation. Also, substantial improvements in psychiatric symptoms have been reported, but the effects on psychiatric symptoms are yet to be established. In the present study, we investigated the effects of single and repeated administrations of psychiatric occupational therapy on psychiatric symptoms and determined whether the effects can be predicted. Our subjects were 215 inpatients or outpatients at our university hospital who participated in psychiatric occupational therapy. Five psychiatric symptoms (i.e. depressive mood, tension, irritability, anxiety and fatigue) were subjectively measured just before and just after each psychiatric occupational therapy by using a visual analogue scale (VAS). As a result, there was a significant short-term effect from single psychiatric occupational therapy, but there was no significant further improvement of any psychiatric symptom from repeated psychiatric occupational therapy. The VAS value at the beginning stage significantly predicted improvement of each psychiatric symptom. These findings suggest that single psychiatric occupational therapy can bring about a short-term effect, whereas repeated psychiatric occupational therapy cannot induce long-term effect (accumulated effect) on psychiatric symptoms, and that the improvement can be predicted by baseline psychiatric symptoms. Copyright © 2011 John Wiley & Sons, Ltd.