. Oral mucositis (OM) is a common adverse reaction associated with chemotherapy. We conducted a six-month longitudinal study to estimate the cumulative incidence of OM during the first six months of chemotherapy in adult patients with cancer other than head and neck cancer. Secondary objectives were as follows: (a) to scrutinize the oral health status of these patients and its evolution during chemotherapy, as assessed by oral health indices; (b) to estimate adherence to prescribed oral hygiene protocol during chemotherapy; and (c) to analyze ulceration-free survival in these patients. . Sixty-four patients participated. Dental health and oral hygiene were assessed at baseline and at the end. Every month, blood tests were performed and oral lesions were recorded. This study was observational, with the only intervention being instruction in the hygiene protocol. The cumulative incidence of OM was estimated with the patient as the unit of analysis. A repeated measures ANOVA was applied to analyze the monthly blood test results. Ulceration-free survival analysis was conducted with adherence to the oral hygiene protocol as a grouping factor, followed by Cox proportional-hazards regression. . The six-month cumulative incidence rate was 43.75%, 95%CI (31.58-56.67%) for OM grade 2 or higher. The hazard ratio of ulceration associated with adherence to the hygiene protocol was 0.154, 95%CI (0.049-0.483), adjusted for age, sex, baseline hygiene index, and class of treatment. Compliance with hygiene recommendations would decrease the OM risk by more than six times, compared to non-compliance.