The effects of four irrigation regimes on fruit production and size were evaluated for three seasons in a mature 'Hass' avocado orchard (Persea americana Mill.). The research was conducted in Chile's Central Valley on a clay loam soil at the surface and sandy loam from 0.3 to 0.9 m. Climate zone is sub-humid, Mediterranean-type. A completely randomized block design was used with four applied water treatments (25, 50, 75, and 100% of reference evapotranspiration, ETo), estimated on the basis of mean daily pan evaporation of the previous week, and four replicates each consisting of six trees. Avocado size and production increased significantly as applied water increased to the level equivalent to 75% ETo. Avocado production differed during the three seasons. For the 2005/2006 and 2007/2008 seasons, the highest yield were 29.6 and 26.9 t ha(-1) with 10,071 and 8887 m(3) ha(-1) season(-1), respectively. The lowest production was observed for the 2006/2007 season when climatic conditions were adverse, and the highest production was obtained with 100% ETo. Production was consistently less in the 'off-crop' years due to alternate bearing for all treatments. However, yield of the 25% and 50% ETo represented only 22% of the yield obtained in the two 'on-crop' years, whereas the yield of 75 and 100% ETo in the off year reached 42% of the average yield of the 2 years of high production. The increasing trend in alternate bearing in the drier treatments emphasizes the need to maintain adequate water supply in avocado even in off-crop years.