The effect of intravenous thrombolysis (IVT) according to etiology and stroke severity in young patients with ischemic stroke (IS) has not been described previously. To analyze the effect of IVT in young patients with IS according to etiological subtype and stroke severity. Observational study with inclusion of IS patients under 55 years of age (2007–2012). Two groups were compared according to IVT treatment. Favorable outcomes were defined as 3 months modified Rankin Scale ≤2. Multivariate analyses were performed to determine those factors independently associated with favorable outcomes, and subgroup analyses were conducted to assess the effect of IVT according to etiological stroke subtype and severity on admission, adjusted for other prognostic variables. We evaluated 262 patients. 63 (24 %) received IVT. The mean age and the sex distribution were similar in the IVT treated and the non-treated groups. Multivariate analyses showed that IVT was associated with a higher probability of favorable outcome (OR, 95 % CI: 4.652, 1.294–16.722) whereas artery dissection (OR, 95 % CI: 0.191, 0.056–0.654) and NIHSS (OR, 95 % CI: 0.727, 0.664–0.797) were associated with a lower probability of a favorable outcome. The subgroup analysis showed that the beneficial effect of IVT on outcomes was significant in moderate-severe strokes (NIHSS ≥8) (OR, 95 % CI: 3.782, 1.095–13.069) and in cardioembolism (OR, 95 % CI: 41.887, 1.001–1751.596). In IS patients under 55 years of age, those with moderate-severe strokes benefit more from IVT than those with mild strokes. Cardioembolic infarctions may benefit more from IV tPA than other etiologies.