Early-onset cataracts are associated with insufficient antioxidative activity, and, therefore, a potential risk of cancer. This study investigated the risk of cancer after being diagnosed with early-onset cataracts. Retrospective claims data from the Taiwan National Health Insurance Research Database were analyzed. Study subjects were comprised of patients with early-onset cataracts, aged 20–55 years (International Classification of Diseases, 9th Revision, Clinical Modification [ICD-9-CM] code 366.00, 366.01, 366.02, 366.03, 366.04, 366.09, 366.17 and 366.18) and newly diagnosed between 1997 and 2010 (n = 1281), and a comparison cohort without the disease (n = 5124). Both cohorts were followed up until 2010 to estimate the incidences of cancer. We used the Poisson regression model to compare incidence rate ratios and the 95% confidence interval (CI). Cox proportional hazards regression was used to assess the hazard ratio (HR) of cancer associated with early-onset cataracts. The overall incidence rate of all cancers was 2.19-fold higher in the early-onset cataract cohort than in the comparison cohort (8.06 vs 3.68 per 1000 person-years) with an adjusted HR of 2.13 (95% CI = 1.48, 3.07). The site-specific analysis also showed a strong relationship, with adjusted HR of 3.24 ((95% CI = 1.30, 8.10) for head and neck cancer, 3.29 (95% CI 1.16, 9.31) for hepatoma and 3.19 (95% CI 1.34, 7.58) for breast cancer. The present study suggests that patients with early-onset cataracts are at an increased risk of being diagnosed with cancer in subsequent years.