Sunflower (Helianthus annus L.) production systems in semi-arid regions often have poor seedling emergence and patchy stands leading to lower yield.  We hypothesized that both plant spacing and early vigor in growth were positively related to individual plant yield, and this relationship would strengthen as water deficits increased.  In a 3-yr study in Colorado, individual plant spacing and associated grain yield was measured for different irrigation treatments.  The third year of data are now being analyzed, but the first two years examining only the role of plant spacing showed a highly significant positive linear and second-order polynomial regressions of individual plant spacing and yield (P < 0.001) for all irrigation treatments in both years.  However, given highly variable individual plant yield with spacing, regression explained no more than 16% of the variation, and there was no support that the relationship strengthened with increasing water deficits.  Based on leverage plots, individual plant spacing explained more of the variation than irrigation treatment.  These results suggest there is a cost to final yield per unit area, where large spacing results in lower yields, even though these plants have the highest yields per plant.