The aim of this study was to assess the impact of diet and behavioural risk factors on caries appearance, and on oral health-related quality of life (OHRQoL) among Portuguese adolescents. An epidemiological study conducted on 782 adolescents between 11-17 years, from randomly selected public schools of the 3rd cycle of basic education. All participants were asked for self-perceived general status health, about tooth-brushing habits and about the using of toothpaste with fluoride and a Food Frequency Questionnaire. The DMFT index (decayed, missing and filled teeth) was evaluated according to WHO criteria. To evaluate the OHRQoL, the 49-items Oral Health Impact Profile questionnaire (OHIP-49) was applied. Consumption more than once a week of tea with sugar, milk with sugar and biscuits were significantly associated with DMFT index. Lower levels in OHRQoL was reported by students who consumed frequently (more than once a week) fast food, chocolate flakes and those who brushed their teeth once a day or less frequently instead of 2-3 times a day. Frequency of consumption of sweetened/fast food was a significant factor associated with caries and quality of life.