The study was designed to identify factors influencing the short term effect of intensive therapy on gross motor function in children with cerebral palsy or developmental delay. Retrospectively, total Gross Motor Function Measure-88 (GMFM-88) scores measured during the first and last weeks of intensive therapy were analyzed (n = 103). Good and poor responder groups were defined as those in the top and bottom 25% in terms of score difference, respectively. The GMFM-88 score increased to 4.67 ± 3.93 after 8 weeks of intensive therapy (P < 0.001). Gross Motor Function Classification System (GMFCS) level (I–II vs. IV–V; odds ratio [OR] = 7.763, 95% confidence interval [CI] = 2.177–27.682, P = 0.002) was a significant factor in a good response to therapy. Age (≥ 36 months; OR = 2.737, 95% CI = 1.003–7.471, P = 0.049) and GMFCS level (I–II vs. IV–V; OR = 0.189, 95% CI = 0.057–0.630, P = 0.007; and III vs. IV–V; OR = 0.095, 95% CI = 0.011–0.785, P = 0.029) were significantly associated with a poor response. GMFCS level is the most important prognostic factor for the effect of intensive therapy on gross motor function. In addition, age ≥ 36 months, is associated with a poor outcome.