Purpose: The objective of this study was to examine the success rate and development of implant stability of photofunctionalized implants with low and extremely low primary stability. Materials and Methods: Primary implant stability was evaluated by measuring implant stability quotients (ISQs) at placement (initial ISQ). Of 301 consecutive implants placed over 2 years, 55 implants (in 38 patients) with initial ISQs less than 60 were included. All implants were photofunctionalized by chairside treatment with ultraviolet light for 15 minutes using a photo device immediately prior to placement, and the generation of superhydrophilicity and hemophilicity was confirmed. The rate of implant stability development was evaluated by calculating the osseointegration speed index (OSI), defined as the ISQ increase per month. Results: Most of the 55 implants (90.9%) were in complex cases and placed in site-developmental sites with simultaneous or staged guided bone regeneration and/or sinus elevation or fresh extraction sites. The success rate was 98.2% after 2 to 3 years of follow-up. The ISQ for all implants was 50.4 ± 7.7 at placement and 74.3 ± 5.7 at stage-two surgery (P .001) after an average of 7.1 months of healing. All ISQ values were greater than 60 at the stage-two surgery. ISQ increases were inversely correlated with initial ISQ (R = 0.789) and resulted in consistently high ISQs at the stage-two surgery regardless of the degree of the initial ISQ. Implants with absent primary stability for which ISQ measurement was not possible at placement also attained ISQs of 75 or greater at the stage-two surgery. The OSIs of low initial stability (55 or lower) implants ranged from 3.9 to 4.7, which was considerably higher than the OSIs of as-received implants reported in the literature (0.36 to 2.8). Conclusion: Photofunctionalized implants placed with low, extremely low, or absent primary stability showed a high success rate, which was corroborated with a rapid increase in implant stability during the healing. Photofunctionalization was more effective for implants with lower primary stability, indicating that the technology was particularly effective in mitigating the challenging conditions. Schlagwörter: hydrocarbon, implant stability quotient, superhydrophilic, superosseointegration, titanium, ultraviolet