BACKGROUND: Follicular lymphoma is an indolent lymphoma that may progress to a highly aggressive form requiring immunochemotherapy. Most regimens utilize rituximab, an anti-CD20 monoclonal antibody, which may affect the clinical course of novel coronavirus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections [coronavirus disease 2019 (COVID-19)]. Here we describe the first case of mild COVID-19 during ongoing oncological treatment without significant deterioration after rituximab administration. CASE SUMMARY: A 74-year-old female with an enlargement of her right palatine tonsil was diagnosed with follicular lymphoma following tonsillectomy and started immunochemotherapy according to the rituximab, cyclophosphamide, vincristine, prednisone regimen. At home before the fourth cycle, she developed nonspecific symptoms (excessive fatigue, loss of appetite and nausea), misdiagnosed as adverse effects of chemotherapy. Unexpectedly, interim positron emission tomography-computed tomography scan, performed shortly before rituximab administration, revealed previously nonexistent pulmonary changes, potentially of infectious etiology. SARS-CoV-2 infection was confirmed by a nasopharyngeal swab (with reverse transcriptase polymerase chain reaction test) performed the following day. Despite rituximab infusion, the patient remained oligosymptomatic and was discharged home for self-isolation. Having reached a negative SARS-CoV-2 status before the subsequently scheduled regimen, the patient successfully received six cycles of rituximab, cyclophosphamide, vincristine, prednisone and obtained complete remission by positron emission tomography-computed tomography. CONCLUSION: Our case shows that rituximab-based immunotherapy due to follicular lymphoma may have no evident negative effect on the COVID-19 clinical course.