A randomized phase III trial comparing watchful waiting with first-line rituximab for advanced stage follicular lymphoma commenced in Japan in December 2016. Watchful waiting is the current standard treatment for stages III and IV (Ann Arbor classification) follicular lymphoma with low tumour burden. This study aimed to confirm the superiority of early administration of first-line rituximab in terms of event-free survival to watchful waiting for stages III and IV follicular lymphoma with low tumour burden. A total of 290 patients will be accrued from 50 Japanese institutions within 5 years. The primary endpoint is event-free survival defined as the period from registration to diagnosis of high tumour burden, therapy using cytotoxic chemotherapy and/or radiotherapy, or death. The secondary endpoints are overall survival, progression-free survival, cytotoxic chemotherapy/radiotherapy-free survival, histological transformation-free survival, response rate, adverse events and serious adverse events. This trial has been registered at the UMIN Clinical Trials Registry as UMIN000025187 (http://www.umin.ac.jp/ctr/index.htm).