Based on exact field experiments, carried out in a Station of Variety Assessment located in south-west Poland in the years 1985-2004, a model was elaborated that indicates how the yield of spring wheat depends on the following factors: number of days with precipitation in the period Mach-July, precipitation in March-April, May, June, July, number of periods without rain longer than 10 days during the vegetation season, complex of soil agricultural value, soil phosphorus and potassium content, nitrogen fertilisation and year of study. The investigation showed that the precipitation factors have a significant effect on the yield level. The dependence of yield on the number of days with precipitation in March-July throughout the whole period was rising. The sum of rainfalls in March-April had a negative effect on spring wheat yield. A reversed situation presented itself in the relation between yield and precipitation total in the successive months. Precipitation in May, June and July had a positive effect on yield, the importance of the July precipitation being, however, smaller than that of the rainfalls of the preceding months. The number of 10-day rainless periods did not affect the yield level. In general, the complex of soil agricultural value turned out to be the factor that affected yield the most. A considerable effect on the size of yield was also exerted by soil phosphorus content and nitrogen fertilisation.