High DDT concentrations and o,p'-DDT/p,p-DDT ratios observed in the air over Taihu Lake, a lake near Shanghai, China, led us to suggest that current use of dicofol in the area north of the lake was the main source of the measured DDTs. To examine this hypothesis, samples of commercially available formulated dicofol in China were collected in 2003 to measure the impurities of DDT related compounds (DOTS). The o,p'-DDT/p,p'-DDT ratio in the samples was 7.0, close to the observed value in the air over Taihu Lake. Average contents of op'-DDT, pp'-Cl-DDT, op'-DDE, and p,p'-DDT in the samples were 114, 69, 44, and 17 g per kg dicofol, respectively. On the basis of a production and distribution survey, total input of DOTS to the environment from the dicofol use in China was estimated to be 8770 t between 1988 and 2002. "Dicofol type DDT pollution", defined as DDT pollution caused by dicofol use and characterized with high op'-DDT/pp'-DDT ratio, might be serious in China, especially in southern and eastern China. The conversion of pp'-Cl-DDT to pp DOE can lead to high pp'-DDE/p,p-DDT ratio and could mislead the evaluation of pp'-DDT resident time in the environment. Therefore, more studies on pp'-Cl-DDT in the environment are needed.