Objectives The growing care demands of an aging population and a smaller workforce is a big societal problem. Therefore, knowledge on how to organize eldercare work without hampering workers` health is needed. We aimed to investigate if workers` daily number of residents cared for over 14 months is associated with low-back pain in eldercare workers.

Methods We included 513 eldercare workers from 122 wards. In each ward, we gathered quarterly data over 14 months on the number of residents, workers, and work schedules and calculated the daily numbers of residents each worker cared for. Workers reported intensity and days with low-back pain via monthly text messages over 14 months. Using generalized linear mixed models adjusted for confounders, we investigated the association between the number of residents workers cared for daily and low-back pain among those workers.

Results In 3-month periods over 14 months, caring for ≥1 resident per day was associated with a 4% [95% confidence interval (CI) 1.02–1.07] increased risk of more days with low-back pain, and a 2% (95% CI 1.00–1.03) increase in low-back pain intensity among workers.

Conclusions Eldercare workers are at a higher risk of experiencing low-back pain during periods when they care for a greater number of residents each day. Maintaining a consistent number of residents and workload for workers over a 14-month period could serve as an effective organizational strategy to prevent low-back pain.