Aim To assess the ability of the patient main problem to predict acuity in adults admitted to hospital wards and step-down units. Background Acuity refers to the categorization of patients based on their required nursing intensity. The relationship between acuity and nurses' clinical judgment on the patient problems, including their prioritization, is an underexplored issue. Method Cross-sectional, multi-centre study in a sample of 200,000 adults. Multivariate analysis of main problems potentially associated with acuity levels higher than acute was performed. Distribution of patients and outcome differences among acuity clusters were evaluated. Results The main problems identified are strongly associated with patient acuity. The model exhibits remarkable ability to predict acuity (AUC, 0.814; 95% CI, 0.81-0.816). Most patients (64.8%) match higher than acute categories. Significant differences in terms of mortality, hospital readmission and other outcomes are observed (p < .005). Conclusion The patient main problem predicts acuity. Most inpatients require more intensive than acute nursing care and their outcomes are adversely affected. Implications for nursing management Prospective measurement of acuity, considering nurses' clinical judgments on the patient main problem, is feasible and may contribute to support nurse management workforce planning and staffing decision-making, and to optimize patients, nurses and organizational outcomes.