We identified two afibrinogenemic girls in two Japanese families and performed molecular analysis to clarify the mechanisms of fibrinogen defects. Genetic analyses were performed by PCR amplification of the fibrinogen gene and DNA sequence analysis. To analyze the mechanisms of mature fibrinogen defects in plasma, we cloned minigenes from the proposita's PCR-amplified DNA, transfected them into CHO cells, and sequenced the cDNA amplified with the RT reaction followed by PCR. Sequence analyses indicated that one was caused by a homozygous 1238 bp deletion of the fibrinogen Aα-chain gene (FGAΔ1238) and the other was a compound heterozygous FGAΔ1238 and novel FGA c.54+3A>C substitution. The minigene corresponding to FGAΔ1238 generates two aberrant mRNAs, both of which may induce a frameshift and terminate prematurely. In contrast, the minigene corresponding to FGA c.54+3A>C generates two aberrant mRNAs, one of which may induce a frameshift and terminate prematurely, and the other uses a cryptic 5' splice site in exon 1, resulting in the deletion of six amino acids in signal peptides. Molecular analyses of both genetic variants suggest that the lack of a mature Aα-chain, impaired assembly, and/or secretion of the fibrinogen molecule may lead to afibrinogenemia.