Context: Active smoking influences normal metabolic status and thyroid function. Objective: To experimentally assess the effects of 1-hour of moderate passive-smoking in a controlled simulated bar/restaurant environment on the metabolism and thyroid hormone levels in healthy non-smokers. Participants: Eighteen (female=9) healthy individuals (mean±std: age=25.3 ± 3.1 yr, height=174.0 ± 10.1 cm, weight=65.2 ± 13.7kg). Design: In repeated-measures randomized-blocks, participants visited the laboratory on two consecutive days. In the experimental condition they were exposed to 1-hour of moderate passive-smoking at a carbon monoxide concentration of 23 ± 1ppm in an environmental chamber, whereas in the control condition participants remained in the same chamber for 1-hour breathing normal atmospheric air. Main Outcome Measures: In both conditions, cotinine serum and urine levels, resting energy expenditure (REE), as well as concentration of triiodothyronine (T₃), free-thyroxine (fT₄), and thyroid stimulating hormone (TSH) were assessed before participants entering the chamber and immediately following their exit. Heart rate and blood pressure were tested in 10-minute intervals during all REE assessments. Results: The mean±std difference of serum and urine cotinine levels (-0.27 ± 3.94 vs. 14.01 ± 6.54 and 0.05 ± 2.07 vs. 7.23 ± 3.75, respectively), REE (6.73 ± 98.06 vs. 80.58 ± 120.91) as well as T₃ and fT₄ (0.05 ± 0.10 vs. 0.13 ± 0.12 and 0.02 ± 0.15 vs. 0.22 ± 0.20) were increased in the experimental compared with the control condition at baseline and follow-up (P < 0.05). No statistically significant variation was observed in the mean difference of the remaining parameters (P > 0.05). Serum and urine cotinine values were linearly associated with REE (P < 0.05). Conclusion: One-hour of passive-smoking at bar/restaurant levels is accompanied by significant increases in metabolism and thyroid hormone levels.