Background Immediate breast reconstruction in 1 stage using permanent implants is gaining popularity and can be performed with or without the use of acellular dermal matrices. This study aimed to investigate the results of breast implants placed submuscularly without acellular dermal matrix and assess the factors affecting surgical complications. Methods From November 2009 to March 2018, 138 patients underwent immediate breast reconstruction with permanent submuscular implants after concomitant skin-sparing or nipple-sparing mastectomies in a single institution. All implants were covered with sufficient soft tissue under a submuscular pocket. Results One hundred thirty-eight patients were enrolled, and a total of 196 breasts were operated. The average age and body mass index (BMI) of the patients were 44.9 +/- 8.8 years and 23.7 +/- 3.6 kg/m(2), respectively. The majority of the mastectomies were therapeutic (81%). The average volume of implants was 389 +/- 89 mL, and the mean follow-up was 33 months. The overall complication rate was 17% (n = 23), with skin necrosis being the most common complication followed by infections. Having a BMI equal to or greater than 25 kg/m(2) was found to be a statistically significant predictor for overall complications (P = 0.002), whereas smoking history, age, and implant volume were not statistically significant. Conclusions Immediate breast reconstruction in 1 stage using permanent implants can be performed with acceptable complication rates and cosmetic outcomes. Our study demonstrated that high BMI is a risk factor for overall complications. With proper patient selection and surgical technique, implants could be completely covered under a submuscular pocket.