The leafhoppers [Asymmetrasca decedens (Paoli) and Empoasca decipiens Paoli] have only recently become significant pests of cotton in Turkey. Action threshold level (ATL) of this pest complex is uncertain. The effect of leafhoppers on fruit set and development and yields of glabrous varieties, Caroline Queen, Cukurova 1518 and SG 125 was compared under sprayed (at the ATL of ten leafhoppers per leaf) or unsprayed conditions in 2002-2003. Comparisons of sprayed and unsprayed cotton showed that leafhoppers significantly reduced numbers of fruit and yield. Total losses of the fruiting parts (square plus boll) were similar, ranging from 35 to 55% as well as cotton losses varied from 35 to 50%. Numbers of sucking insects (cotton aphid, whitefly and thrips) and also lepidopterian pests (pink bollworm and bollworm) were very low in both years, and they did not affect cotton development. The current regional ATL for the cotton leafhoppers appears to be too high, and it appears that even five leafhoppers per leaf may be sufficient to significantly reduce yields. It is concluded that ATL of the cotton leafhoppers needs to be re-evaluated in cotton fields in Turkey and also other geographic regions which share similar ecological conditions and same leafhopper species.