Using data from 266 age- and sex-matched pairs of Jamaican children with autism spectrum disorder (ASD) and typically developing (TD) controls (2-8 years), we investigated whether glutathione S-transferase theta 1 ("GSTT1") modifies the association between blood manganese concentrations (BMC) and ASD. After adjusting conditional logistic regression models for socioeconomic status and the interaction between "GSTT1" and "GSTP1" (glutathione S-transferase pi 1), using a recessive genetic model for "GSTT1" and either a co-dominant or dominant model for "GSTP1," the interaction between "GSTT1" and BMC was significant (P = 0.02, P = 0.01, respectively). Compared to controls, ASD cases with "GSTT1"-DD genotype had 4.33 and 4.34 times higher odds of BMC > 12 vs. [less than or equal to] 8.3 µg/L, respectively. Replication in other populations is warranted.