Objective: To identify the oral manifestations and salivary pH before and after chemotherapy in pediatric patients. Material and Methods: The sample consisted of 46 children. It was included children from 2 to 12 years, diagnosed with Acute Lymphoblastic Leukemia (ALL) and they would be subjected to chemotherapy. The first evaluation was performed before chemotherapy and consisted of anamnesis, oral clinical examination and saliva collection to measure the pH. The second evaluation was performed among 10 to 15 days after the start of chemotherapy treatment and consisted of new saliva collection and clinical examination. Results: The results showed that the gender most affected by childhood cancer was male. The predominant age group were children aged 8-12 years. The most frequent oral manifestations were mucositis (35%), xerostomia (26%), toothache and dysphagia (22%). There was also a significant reduction in salivary pH after chemotherapy, p <0.0001, indicating that the average pre and post chemotherapy are different (Δ = 10.6) with statistical significance at the 5% level. Conclusion: Patients with ALL can present any lesion in the oral cavity during or after the start of chemotherapy and undergo changes in the amount of salivary pH. The dentist needs and know the oral manifestations and intervene in the oral health of patients with ALL, contributing and assisting in their treatment.