Both obesity and diabetes mellitus are well-known risk factors for tendinopathies. We retrospectively compared the efficacy of single corticosteroid injections in treating lateral epicondylitis in patients with and without metabolic syndrome (MetS). Fifty-one patients with lateral epicondylitis and MetS were age- and sex-matched with 51 controls without MetS. Pain severity, Disability of the Arm, Shoulder, and Hand score, and grip strength were assessed at base line and at 6, 12 and 24 weeks post-injection. The pain scores in the MetS group were greater than those in the control group at 6 and 12 weeks. The disability scores and grip strength in the MetS group were significantly worse than those of the control group at 6 weeks. However, there were no significant differences at 24 weeks between the groups in terms of pain, disability scores and grip strengths. After 24 weeks, three patients (6%) in the control group and five patients (10%) in the MetS group had surgical decompression (p = 0.46). Patients with MetS are at risk for poor functional outcome after corticosteroid injection for lateral epicondylitis in the short term, but in the long term there was no difference in outcomes of steroid injection in patients with and without MetS.