Background. The workload of nurses in the intensive care unit (ICU) can affect the quality of nursing services. Aim. This study aimed to determine the relationship between the nursing activity score and missed care in patients hospitalized in the ICU in Zanjan, Iran. Methods. This observational and prospective study was conducted from April 3 to September 18 in 2021. The study utilized a patient and nurse profile questionnaire, the Nursing Activity Score (NAS), and a checklist for missed care as research tools. Missed care was observed in the 301 patients for whom the NAS was calculated. Analysis of variance (ANOVA) was used to investigate the differences in mean levels of missed care. Logistic regression models were used to assess the association between factors and missed nursing care. Results. Results show that the medical ICU’s mean NAS was 76.31 (95% CI: −13.06–14.89). In 9 dimensions of care, the extent of missed care was 40.7%. In the care dimensions of assessment, hand hygiene, and infection control, the mean NAS had a statistically significant increase at higher levels of missed care ( P < 0.5). Furthermore, work experience was identified as a protective factor for missed care (OR = 0.59, 95% CI: 0.37–0.94, χ2 = = 4.97, p = 0.026). Conclusion. The study revealed a high incidence of missed care. The study revealed that the mean workload was high in certain dimensions of care such as assessment, hand hygiene, and infection control. The increase in workload for nurses results in lost care. Nonetheless, the utilization of experienced nurses can help mitigate this problem. However, utilizing experienced nurses can help reduce this problem.