Comparing unit cost variation between departments and reasons for consultation in outpatient health services provided by a social security company from Querétaro, Mexico. A study of costs (in US dollars) was carried out in outpatient health service units during 2004. Fixed unit costs were estimated per department and adjusted for one year's productivity. Material, physical and consumer resources were included. Weighting was assigned to resources invested in each department. Unit cost was estimated by using the micro cost technique; medicaments, materials used during treatment and reagents were considered to be consumer items. Unit cost resulted from adding fixed unit cost to the variable unit cost corresponding to the reason for consulting. Units costs were then compared between the medical units. Unit cost per month for diabetic treatment varied from 34.8 US dollars, 32,2 US dollars to US 34 US dollars, pap smear screening test costs were 7,2 US dollars, 8,7 US dollars and 7,3 US dollars and dental treatment 27 US dollars, 33 US dollars, 6 and 28,7 US dollars. Unit cost variation was more important in the emergency room and the dental service.