Background: Cigarette smoking increases the risk of chronic diseases; heating instead of burning tobacco can lower these risks. This study (with 984 adult American smokers) examined whether favorable changes occur in eight co-primary endpoints (HDL-C, WBC, FEV1%pred, COHb, Total NNAL, sICAM-1, 11-DTX-B2, 8-epi-PGF2α) indicative of biological and functional effects when cigarette smokers switch to the heat-not-burn Tobacco Heating System 2.2 (THS). Additionally, these biomarkers of exposure (BoExp) were quantified: MHBMA, 3-HPMA, Total NNN, CEMA, 3-OH-B[a]P, HMPMA, Total 1-OHP; NEQ, and CO exhaled. Methods: Participants were randomized to continued smoking of their preferred cigarette brand (n = 496) or to using THS (IQOS brand) (n = 488) for 6 months. THS has a maximum heating temperature of 350°C, delivering 1.21 mg nicotine/stick and 3.94 mg glycerin/stick under the Health Canada Intense smoking regimen. Results: The main outcome was a favorable change 6 months after baseline, with statistically significant improvements in 5 out of 8 biomarkers of effect (HDL-C, WBC, FEV1%pred, COHb, Total NNAL) when smokers switched to THS compared with those who continued to smoke cigarettes. Likewise, BoExp were markedly reduced. Conclusions: All endpoints showed favorable changes in the same direction as with smoking cessation and improved biological effects were observed in smokers who predominantly used THS compared with continued cigarette smoking, with similar nicotine levels in both groups. Impact: Improvements in 5 of 8 biomarkers of effect are supportive of the research hypothesis, suggestive of disease risk reduction potential for smokers switching to THS instead of continuing to smoke cigarettes.