The result series of our irrigation experiment, made between 1977-1997, using two tomato varieties, can demonstrate the effect of different cultivation and environmental factors on yield. The yields of two studied tomato varieties are presented by the different fruit size, yield results of 4 different water supplies and yield results during 21 years. According to the average results of these 21 years, the increasing of yield quantity was observed in case of two varieties thanking to the irrigation, which proves the necessity of regular irrigation. According to the yield quantity of non irrigated plants and the effect of irrigation different crop year categories can be assigned. In most of the years (app. 55-66 %) regular irrigation is required but in part of the years (20-25 %) irrigation can be ineffective. The accumulated amount of rainfall during growing season, taken as function of accumulated amount of average temperature, can suitably present the environmental conditions of tomato and also notice the expectable crop results. Irrigation can be more important between 20 June and 10 August.