Studies of nurses and other health professionals indicate that autonomy is an important determinant of job satisfaction and turnover. This study analyzed selected characteristics of hospital nursing units to identify those features of the work setting that influence staff nurses' perceptions of autonomy; comparisons among nurses who work in different clinical areas were made. Data were collected by interviewing 789 nonsupervisory registered nurses who were employed full time at one large university-affiliated hospital. Personal and job-related information was obtained for each nurse. Structural features of units, such as workload, were gathered from head nurse reports and hospital records. Findings indicated that nurses' perceptions of autonomy are influenced by both personal characteristics of the nurse and structural features of the units. The nurse's sense of personal efficacy and the relationship she has with her head nurse are two important determinats of autonomy across all units. Workload, primary nursing, and staffing patterns are influential factors in predicting autonomy for nurses who work in critical care areas. Implications of these findings for nursing administrators are discussed.