Rapid improvements in lean growth potential, health status and management of the weaned pig have resulted in changes in the energy and amino acid requirements such that previous recommendations or diet formulations used in the past may no longer result in maximum performance. In addition, recent research showing increases in nursery exit weights will double by marketing places even greater importance on maximizing weight gain of the weanling pig. So, we ask the question: .How much lysine is enough and how much is too much, and how much lysine in the starter is economical? Currently, recommended lysine:energy ratios vary from as low as 2.8 to as high as 5.0 g total lysine/Mcal DE for pigs up to 25 kg. In 1998, the National Research Council (NRC) recommended 3.5 g total lysine/Mcal DE for pigs above 5 kg live weight. Two experiments were conducted to determine the optimum ratio of total lysine/ DE for the weaned pig. In experiment 1, a total of 240 pigs at weaning were fed a standard commercial SEW diet for 5 days followed by a transition diet for 8 days. Pigs were then fed the experimental diets for a 28-day period starting on day 13 postweaning. The six experimental diets contained total lysine/DE ratios of 2.7, 3.0, 3.3, 3.6, 3.9 or 4.2g total lysine/Mcal, equal to 0.95%, 1.05%, 1.16%, 1.26%, 1.37% and 1.47% total lysine, respectively. Growth rate and feed efficiency increased with increasing lysine/DE ratio. There was no effect of lysine/DE on feed intake. Because growth rate increased linearly to the highest level (4.2 g/Mcal; 1.47 % total lysine), we concluded that the required lysine/DE ratio for the weaned pig is at or above this level and thus may be higher than most literature values and recommendations. A second experiment was therefore necessary. In experiment 2, a total of 240 pigs were fed two levels of DE; low energy (LE, 3.4 Mcal DE/kg) or high energy (HE, 3.6 Mcal DE/kg), at 5 lysine/DE ratios (3.7, 4.0, 4.3, 4.6, and 4.9 g total lysine/Mcal). Thus, 10 diets, ranging in total lysine content from 1.26% to 1.76%, were investigated. The lysine/DE ratios were selected to bracket existing literature values for the weaned pig and included higher values than those used in experiment 1. Pigs were fed one of the 10 experimental diets starting on day 7 post-weaning for a 28 day period. Growth rate increased and feed efficiency improved with increased lysine:DE ratio. Overall, the level of DE did not influence growth rate. However, pigs fed the high-energy diets ate 13% less feed than those fed the low-energy diets in week 1 and had 12% lower growth rate. Overall feed intake was 4% lower on the high-energy diets, but DE intake was similar for both DE levels. The lysine:DE ratio that maximized performance was calculated from the regression equation. We concluded that for pigs growing from 7.5 to 22.5 kg, the requirement is 4.65 g total lysine/Mcal DE, equal to 1.60% total lysine. The existing recommended ratios for the weaned pig are inadequate to support its full growth potential, so the results of experiment 1 were confirmed. Increasing the lysine level of a starter diet from 1.30% to 1.60% will cost about $80 per tonne using current feed costs. With actual total feed intake of pigs on lysine:DE levels for this period, the feed cost per pig will increase by $1.86 at the optimum lysine level. Assuming this additional lysine increases nursery exit weights by 1 kg, and assuming, based on previous research at the Prairie Swine Centre, that this improvement will increase weights at 20 wk of age by 2 kg, we calculated the change in net income per pig by using ratios above 3.7 g/Mcal. The maximum benefit is obtained at 4.30 g/ Mcal (1.5%) when hog market prices are above $1.75 (Table 3), but lowers levels of 1.3% to 1.4% optimized financial returns at lower market prices. Clearly, hog market prices dictate the benefit of using optimum versus maximum lysine:DE ratio in the starter diet and provide further support for the logic that feeding programs must be developed in the context of changing economic conditions.