Outpatient care accounts every year for a large share of the National Health Fund spending, however characteristics of supply have not been thoroughly investigated. Objective of the study is the description of the outpatient care system of Lazio region and of the main characteristics of outpatient clinics, through indicators obtained using data from the Outpatient Care Information System (SIAS) for 1999. Outpatient clinics were classified into three categories: ASL managed clinics, private clinics and hospital trusts. Absolute and relative density of supply (respectively DAO and DRO) were used as indicators of clinics distribution in the regional area. Number of specialties, average procedure weight and volume of procedures performed were used as indicators of complexity. Absolute density of supply (DAO = n. of dispatch points/population) is generally high, and a large and statistically significant variability is observed (p < 0.001). The relative density (i.e. the correlation coefficient between DAO and population density) is positive overall (r = 0.43), but it is higher within the private sector (r = 0.62) independently from provider category. Statistically significant differences were observed among categories of providers in terms of average number of specialties (ASL managed: 9.9; private clinics: 1.7; hospital trusts: 16.1), average weight (ASL managed: 1.1; private clinics: 0.9; hospital trusts: 1.3) and average volume of procedures supplied (ASL managed: 35.000; private clinics: 59.000; hospital trusts: 282.000). The administrative SIAS database was a useful tool to define indicators aimed at describing characteristics of the outpatient care system, although these results must be confirmed with a higher and more homogeneous level of coverage.