Aim: to assess space changes in the dental arch following premature extraction of the first primary lower molar in 7-9 years children. Subjects and methods: The study consisted of 48 children in the age group of 7-9 years old who attended Pediatric Dentistry Department- Faculty of Oral and Dental Medicine- Cairo University and underwent premature extraction of primary mandibular first molar. Clinical and radiographic examinations were performed to confirm the indication of first primary molar extraction. The degree of successor root development was assessed according to Nolla’s system with the aid of radiographic images. Impressions were taken immediately before the extraction and initial casts were poured. All patients had recall appointments every 2 months and longitudinally, the procedure of impression taking and cast preparing was repeated at each follow-up at two, four, six and eight months after extraction. Thus each patient was subjected to clinical evaluation at five stages. Measurements on the casts including arch length, arch perimeter, inter-molar width, inter-canine width and (D+E) Space were performed to assess space changes initially before extraction and at each follow-up interval. Any needed dental treatment was delivered to our patients. Results: The (D+E) space was decreased by (0.64±0.59) at the end of follow-up period which was considered statically significant (P = 0.001). However, the clinical significance of such a value is questionable. The arch length, arch width and arch perimeter showed a statistically insignificant difference between the initial examination and the 8 month follow-up examination (P=1). An inverse relationship was found between the amount of space loss at the end of follow up period and degree of successor’s root development according to Nolla’s system. Conclusions: The premature loss of the lower first primary molar in the mixed dentition where permanent first molars were erupted and in occlusion resulted in statistically significant space loss at the (D+E) space. Although 0.64 mm space loss was considered statistically significant, it is of limited clinical significance in most clinical situations. The greatest space changes were observed in the first four months after the premature extraction. There were no significant changes in the arch length, arch perimeter, inter-molar width and inter-molar canine width during the experimental period of eight months after extraction, suggesting that mesial movement of permanent molars probably did not occur.