Background: The present retrospective evaluation study examined treatment efficacy in preschool children with severe specific language impairments. Besides the efficacy of 2 inpatient treatment conditions, differing in periods and intensity of parent advice, will be compared. Material and Methods: 106 children (mean age: 72.1; SD 9.1 months) received 6 weeks of treatment with a traditional therapy en bloc (EB) and 78 children (mean age: 72.3; SD 10.4 months) with a new therapy in 3 intervals (I) respectively. Receptive language abilities, expressive vocabulary size and nonword repetition were utilized as language measures at the beginning and end of treatment. Results: In both treatment conditions, the children, on average, made significant gains in the outcome measures (p < 0.0001). En bloc-like interval therapy yielded big effects in receptive language abilities (EB: d = 0.89; 95 %-CI: 0.72-1.05 vs. I: d = 0.95; 95 %-CI: 0.71-1.19). Medium effect sizes (EB: d = 0.60; 95 %-CI: 0.48-0.72; I: d = 0.79; 95 %-CI: 0.61-0.98) resulted in expressive vocabulary, but rather minor ones in phonological processing (EB: d = 0.37; 95 %-CI: 0.22-0.52; I: d = 0.48; 95 %-CI: 0.28-0.67). No signifi cant post-intervention difference between the therapy modes was only observed in receptive language (95 %-CI of the difference at the end of treatment: -1.85-5.17). Conclusions: An intensive multidisciplinary approach with preschool children is justified when the children suffer from severe deficits in language comprehension and expressive vocabulary even after sufficient outpatient treatment.