Aim: To assess the validity of the Waterlow screening tool in a cohort of internal medicine patients and to identify factors contributing to pressure injury. Method: A longitudinal cohort study design was used. A total of 274 patients (mean age 65.3 years) admitted through the emergency department or outpatient clinics of a tertiary hospital in Brisbane, Australia, and expected to remain in hospital for at least 3 days were screened on admission using the Waterlow screening tool. Their pressure ulcer status was monitored and recorded every second day. The main outcome measure was pressure ulcer incidence. Results: Fifteen participants (5.5%) had an existing pressure ulcer and a further 12 (4.4%) developed a pressure ulcer during their hospital stay. Sensitivity of the Waterlow scale was 0.67 (95% confidence interval [CI]: 0.35-0.88), specificity was 0.79 (95% CI: 0.73-0.85), positive predictive value was 0.13 (95% CI: 0.07-0.24) and negative predictive value was 0.98 (95% CI: 0.94-0.99). Conclusion: This study provides further evidence of the poor predictive validity of the Waterlow scale. A suitably powered, randomized controlled trial is urgently needed to provide definitive evidence about the usefulness of the Waterlow scale compared with other screening tools and with clinical judgment.