Introduction: Length of stay (LOS) is an indicator of resource utilization in hospitals. In this survey, we have studied the patients' LOS and its influential clinical and nonclinical factors in a gynecology and obstetrics hospital affiliated with Tehran University of Medical Sciences.Methods: This survey is a cross sectional descriptive-analytical study. We reviewed 3421 inpatient charts in oncology, surgery and obstetrics units in 2008. The required data were collected using a data collection sheet and inpatients interviews. Data were analyzed using Kolmogrov-Smirnov, Mann-Whitney, Kruskal- Wallis, and Spearman correlation tests.Results: The median of LOS in the studied hospital was 50.8 hours and 48.5, 54.4 and 94.2 hours in obstetrics, surgical and oncology units, respectively. The factors which increased LOS were being single, having worker or farmer & stockbreeder or retired husbands, rural insurance coverage, admission in oncology unit, admissions on Wednesdays and Thursdays, being admitted by internists and by residents, relative recovery or need to follow up at discharge time, elective admission, the number of performed diagnostic experiments, and diseases such as gastrointestinal, neoplastic or endocrine diseases (all p values<0.005) .Conclusion: Among influential factors, policymakers and managers can only change the admission days and the number of diagnostic experiments in order to decrease LOS. Thus, they should prevent admissions in last days of week, except emergent admissions, and primary diagnostic experiments should be performed before admitting into the hospital to be able to use hospital beds and other resources properly.