On 26th December 2003, a severe earthquake hit the city of Bam in Kerman province, southern Iran. It destroyed around 90% of houses and at least 60% of the public buildings, and claimed the lives of more than 20,000 persons. To investigate whether acute stress caused by the Bam earthquake could alter the sex ratio at birth (SRB) 6–12 months later, the present study was done. The number of live births by sex was obtained from the National Organization for Civil Registration (Kerman province). The SRB was expressed as the male proportion. A prominent decline in the SRB (̃0·467) 11 months after the earthquake was observed (χ2=6·68, df=1, p=0·009). There was no significant difference between Bam and Kerman province (excluding Bam) for SRB (χ2=0·44, df=1, p=0·51) for a period of 33 month before the earthquake (from April 2001 to December 2003). It might be concluded that psychological tensions and stress are associated with a decrease in SRB.