OBJECTIVE To test the hypothesis that pressure sores significantly influence the nursing workload, after taking confounding factors into account, i.e., patients' characteristics simultaneously associated with the occurrence of pressure sores and the nursing-time requirements. METHODS Retrospective analysis of administrative data for a cohort of long-term-care residents (Planification Informatisée des Soins Infirmiers Requis database). Two methods were used to control for confounding factors: analysis of covariance and sample restriction. METHODS Long-term-care institutions of Quebec, except exclusively psychiatric and private centers. METHODS Data was available for the 13,555 residents aged 65 or more whose health status changed during the year 1993-1994. RESULTS Prevalence of sores was 4.0% (544/13,555). Before any adjustments were made, residents with pressure sores needed, on average, 63 minutes more than the residents without pressure sores. Two confounding factors were identified: dependence in the activities of daily living and physical mobility. Analysis of covariance showed that the adjusted increase in daily nursing care was 19 minutes. In the second analysis, a restricted homogeneous sample for the confounding factors was used (5,849 patients, including 414 patients with pressure sores). According to this method, the adjusted increase in daily nursing care was 17 minutes. In both analyses, the increase was noticeable in the spheres of alimentation, mobilization, and treatments. CONCLUSIONS The presence of pressure sores significantly influences the nursing workload, even after eliminating the influence of confounding factors. Because nursing time can be translated into cost, effective prevention strategies and strategies of reducing the costs of treating sores should be analyzed.