Eight six-year old German children with development disabilities regarding such number competencies as have been demonstrated to be among the most relevant precursor skills for the acquisition of elementary mathematics received intensive training with the program "Mengen, zählen, Zahlen" ["quantities, counting, numbers"] (MZZ, Krajewski, Nieding, & Schneider, 2007). In 24 30-minute training sessions administered across an interval of eight weeks, four children received the full program. Using a pretest-posttest control group design, the effectiveness of the training was demonstrated with a large effect size (d = 4.6). In addition, a shortened training of the waiting control group (n = 4) after finishing the principal evaluation replicated the appropriateness of the MZZ program (d = 2.5) as a means of remediating early number competence deficiencies at the onset of formal schooling.