Previous time series or panel studies of asthmatics have reported respiratory health effects following short-term exposure to ozone (O-3). We followed 186 children aged 10 years old in Athens and Thessaloniki, Greece for 5 weeks during the academic year 2013-2014 and recorded daily their respiratory symptoms, absenteeism and peak expiratory flow (PEF). We applied mixed models controlling for various possible confounders to investigate the daily associations between O-3 exposure - derived from weekly personal and fixed school site measurements calibrated using daily values of the fixed monitoring site nearest to the child's school location -and PEF, presence of any symptom, cough and stuffy nose, as well as absenteeism. We tested the robustness of our findings to varying modeling assumptions and confounders and investigated effect modification patterns by medication use, time spent outdoors and prevalence of asthma. A 10 mu g/m(3) increase in O-3 personal exposure was associated with increased odds of any symptom (odds ratio (OR): 1.19, 95% confidence interval (CI): 0.98, 1.44), largely attributed to the increase in the odds of stuffy nose (OR: 1.23, 95% CI: 1.00, 1.51). PEF and absenteeism were not related to O-3 exposure. Our results were robust to several sensitivity analyses. Effects were modified by medication use as presence of symptoms but also decreases in PEF were mainly reported among non-users, while our effect estimates were not driven by the asthmatic subgroup of children. Our findings indicate that short-term O-3 exposure may be associated with respiratory symptoms extending previously reported results for asthmatics to the general population.