Purpose To investigate the incidence of, and predictive factors for, a subsequent hernia repair on the contralateral side following a primary unilateral hernia repair. Methodology A prospective study from the Swedish Hernia Register based on 151,297 patients undergoing unilateral inguinal hernia repair using an open or endo-laparoscopic technique, during 2007-2019. Incidence, predictive factors for, and time to, a contralateral hernia repair were analyzed. Results A contralateral hernia repair was performed in 7.4% within 2.7 years (median) after the index operation. The median follow-up time was 7.5 years. Significant predictors were male sex, high age, medial inguinal hernia, combined inguinal hernia, hernia defect size 1.5-3.0 cm and > 3 cm, and a repair on the left side. Endo-laparoscopic repairs and obesity were associated with a lower incidence of subsequent contralateral repair. Conclusion The risk for a following contralateral hernia repair is 7.4%. Pre- and perioperative findings: medial or combined hernia; a large hernia defect size; and a left-sided hernia, support a proceeded perioperative contralateral exploration and a peri-operative bilateral repair.