Type 2 fiber atrophy is a non-specific phenomenon in several diseases, including rare cases of congenital myopathies. These myopathies manifest early in life, and are characterized by hypotonia and muscle weakness. In this report, we describe a morphometric analysis of type 1 (T1F) and type 2 (T2F) fibers in three children (0.8, 4 and 8 years old) with signs of myopathy and T2F atrophy and assess the extent of T2F involvement compared to T1F. We measured the lesser diameter of T2F and T1F processed for ATPase activity at pH 9.4, in order to determine the pattern of muscle involvement. A histogram of the frequency of muscle fiber lesser diameter was used to compare the results with other reports in the literature. Statistical analysis using the chi-square test for goodness of fit showed that the lesser diameter of T2F differed among the children (p<0.05). Hence, it was not possible to determine a pattern of T2F atrophy based on the frequency of fiber size distribution in these cases of myopathies.