Purpose Chemotherapy-induced oral mucositis (CIOM) is a common side effect of cancer therapy that may lead to significant morbidity and interfere with the treatment plan. The present prospective, cross-sectional study intended to describe the prevalence of clinically relevant CIOM (CRCIOM) in outpatients receiving chemotherapy for solid tumors. Methods Intra-oral assessments were performed on 298 consecutively recruited patients, who had undergone at least 14 days of chemotherapy for solid tumors in our outpatient oncology department. The presence of CIOM was evaluated using the Oral Mucositis Assessment Scale. CRCIOM was defined as the presence of ulcers (=1 cm2), severe erythema, and/or inability to eat solid foods (WHO grades 2-4). Furthermore, the current levels of oral hygiene and oral health were measured. Results A low prevalence (18 patients, 6%) of CRCIOM was found in the investigated patient collective, including 1% of patients with severe (WHO grade 3/4) CIOM. In the CRCIOM group, 16 patients were male, and two were female; 8 patients with CRCIOM had received head and neck radiotherapy. A higher prevalence of CRCIOM was found in smoking patients (12.7% vs. 4.5%, p?<?0.05) and in the patients who have not had a dental checkup within the preceding 12 months (11.2% vs. 3.0%, p? <?0.01). Diabetes mellitus and low WBC appeared not to be associated with higher CRCIOM rates. The plaque and gingival indexes were significantly increased (p?<?0.01) in the CRCIOM group. Conclusions Although CRCIOM was a rare event in the investigated patient population, our results emphasize that pre-treatment dental therapy and primary preventive measures (including oral hygiene instructions) can be improved. Before starting chemotherapy, increased awareness of individual risk factors, such as male sex, tobacco smoking, low dental checkup frequency, poor oral hygiene, and a reduced oral health status, could help to prevent CRCIOM.