This study was conducted to determine effect of deficit irrigation on yield for lettuce grown under unheated greenhouse condition. The research was carried out at the Agricultural Research Station of Yenisehir High School of Uludag University in Bursa, Turkey, in 2007. In the study, water was applied to lettuce as 100, 75, 50, 25 and 0% (as control) of evaporation from a Class A Pan corresponding to 2 day irrigation frequency. Irrigation water applied to crops ranged between 51.4 and 484.5 mm, and water consumption ranged between 78 and 511 mm. There were significant differences between treatments in yield, plant height, head diameter, dry matter and mean leaf number. The highest yields were 15.10 and 12.05 t ha(-1) for the KIcp and K2(cp) treatments, respectively. Crop yield response factor (k(y)) was found as 1.096. The highest values for water use efficiency (WUE) and irrigation water use efficiency (IWUE) were found to be 2.96 and 2.70 kg mm(-1) for the K2(cp) treatment. Under the conditions that water resources are scarce, it can be recommended that K2(cp) treatment is most suitable as a water application level for lettuce irrigation by drip irrigation under unheated greenhouse condition.