Upper respiratory tract infections (URIs) are the most common infections worldwide. Their frequent inappropriate treatment with antibiotics is likely to increase antibiotic resistance, contribute to morbidity and mortality, and waste scarce resources. Using data from registration books and prescriptions, we measured patterns and assessed appropriateness and predictors of antibiotic prescribing for viral and bacterial URIs treated in health centers located in two slum communities in Bangkok, Thailand. Based on recorded diagnoses and symptoms, 91% of the patients probably had viral URIs; 60% of viral and 89% of bacterial URI patients were prescribed an antibiotic. Compliance with the national treatment guideline was 36.4% for treatment of viral URIs and only 1.7% for treatment of bacterial URIs. Amoxicillin was the most frequently prescribed antibiotic regardless of diagnosis. Among viral URI patients, those who were young, male, and self-paying were more likely to receive antibiotics; part-time physicians were more likely to prescribe antibiotics for these patients. Among patients with bacterial URIs, those who paid for drugs by themselves were more likely to receive antibiotics compared to patients covered by the national health insurance plan. We used these formative results as input to the design of health center and community interventions to encourage more appropriate prescribing for URI among adults.