In the present study we investigate decision making under ambiguity and decision making under risk in Parkinson's disease (PD) patients without cognitive impairment and in patients affected by Parkinson's disease dementia (PDD). In decisions under ambiguity, participants are not aware of the rules for gains and losses and have to learn about the utility of their selections through feedback. The two patient groups showed significant deficits and did not differ in the frequency of advantageous choices, though they had a markedly different cognitive profile. In decisions under risk, explicit information on the options' probabilities as well as on the associated gains and losses is given. PD patients and healthy controls performed at the same level, whereas PDD patients made significantly more risky and disadvantageous decisions. Results of the study suggest that both patient groups are impaired in decision making when learning by feedback and emotional processing is required, while only the PDD group shows difficulties when decision making is based on cognitive reasoning strategies.