This paper is one of two that explores relationships between nursing staff resources, ward organizational practice and nurses' perceptions of ward environments. Here we examine relationships between staff numbers, care organization and nursing practice. A subsequent paper examines the effects of grade mix and staff stability. The data were collected in the mid-1990s from a nationally representative sample of 100 acute hospital wards and 825 nurses. Our analyses provide important insights for managers seeking to achieve the strategic aims set out in Working Together, and subsequent National Health Service (NHS) policy placing emphasis on making the best use of nurses, focussing on mobilising their knowledge, skills and talents within the context of extended work roles. Findings show that number of nurses is associated not only with ward organizational systems but also with a range of care processes and staff outcomes which hitherto have been linked only to ward organizational systems. The latter have been identified as providing different opportunities for developing multidisciplinary working and nurses' autonomy but the central importance of having an adequate number of staff to achieve these aims has been largely overlooked in research. Having fewer nurses was associated with both hierarchical ward organizational structures and hierarchical attitudes to care provision, denoting lower standards of nursing practice. A larger nursing complement was significantly associated with devolved organizational structures but no link was established between more staff and higher standards of nursing practice. When there were more staff there was a positive association with nurses' perceptions of multidisciplinary collaboration, their ability to cope with workload and job satisfaction. Employing an adequate number of nurses to provide care is clearly beneficial for nurses themselves. Patients are also likely to benefit from a lower incidence of 'hierarchical practice' associated with having a low nurse/bed ratio within a ward.