In this paper, analytical results obtained from 174 frying oil samples collected from 131 fast food shops and restaurants by Food Inspection Services, are summarized. Quantitation of polar compounds indicated that about 35% of the samples had alteration levels higher than the limit established in the Spanish regulation for discarding frying fats (25% polar compounds). High levels of oxidized triglycerides and polymeric compounds were found in a significant number of oils which demonstrated the need for improving frying oil quality in this sector.