Objectives: The primary goal of this study is to examine the level of distress, feelings, and level of functioning of oncology patients associated with specific symptoms of malignant disease and treatment side effects. Study design: A cross-sectional study was conducted. Subjects and methods: The research includes 103 patients treated in the Department of Oncology and Nuclear Medicine at the Sestre milosrdnice University Hospital Center in Zagreb. The research instrument was an anonymous questionnaire, the Croatian version of a validated scale for measuring distress in oncology patients called “The Distress Thermometer”. Results: The largest percentage of participants, 43%, did not have a fear of treatment side effects. Twenty-four percent of patients had a fear of nausea and vomiting, and 11% had a fear of hair loss. A total of 94.2% of participants reported having sufficient psychological support from their loved ones. The average level of distress (arithmetic mean) among participants on a scale from 0 to 10 was 3.61. Concerns of a physical nature were expressed by 31% of participants, who worried about the loss or changes in their physical abilities, while 20% were concerned about fatigue. Emotional concerns were expressed by 31% of participants, who were worried about feeling worthless or being a burden to others, while, as a concern of social nature, 50% were anxious about their relationships with family members. There was no significant difference in the level of distress based on gender (P = 0.45). Conclusion: Overall, participants are not afraid of chemotherapy side effects, and their level of distress is not significantly pronounced. However, specific concerns of physical, emotional, and social nature were observed. Participants are satisfied with the psychological support they receive from their families.