BackgroundInadequate nurse staffing has been reported to lead nurses to omit required nursing care. In South Korea, to reduce informal caregiving by patient families and sitters and to improve the quality of nursing care, a public hospital operated by the Seoul Metropolitan Government has implemented a policy of increasing nurse staffing from 17 patients per registered nurse to 7 patients per registered nurse in 4 out of 13 general nursing units since January 2013.AimThe study aims to compare missed nursing care (omission of required care) in high‐staffing (7 patients per nurse) units vs. low‐staffing (17 patients per nurse) units to examine the effects of nurse staffing on missed care.MethodsA nurse survey conducted in July 2013 targeted all staff nurses in all four high‐staffing and all nine low‐staffing units; 115 nurses in the high‐staffing units (response rate = 94.3%) and 117 nurses in the low‐staffing units (response rate = 88.6%) participated. Missed nursing care was measured using the MISSCARE survey that included 24 nursing care elements. Nurses were asked how frequently they had missed each element on a 4‐point scale from ‘rarely’ to ‘always’.ResultsOverall, nurses working in high‐staffing units had a significantly lower mean score of missed care than those in low‐staffing units. Seven out of 24 nursing care elements were missed significantly less often in high‐staffing (vs. low‐staffing) units: turning, mouth care, bathing/skin care, patient assessments in each shift, assistance with toileting, feeding and setting up meals.ConclusionThe findings suggest that increasing nurse staffing is associated with a decrease in missed care. Less omission of required nursing care is expected to improve nursing surveillance and patient outcomes, such as patient falls, pressure ulcers and pneumonia.Implications for nursing and health policyAdequate nurse staffing should be ensured to reduce unmet nursing needs and improve patient outcomes.