A total of 180 mixed-sex pigs (Duroc × (Yorkshire × Landrace); average initial body weight of 7.36 ± 0.2 kg) weaned at 21 ± 1 days were fed corn-soybean meal-wheat-based diets to determine the optimal standardized ileal digestible (SID) tryptophan to lysine ratio (Trp : Lys) in a 2 × 5 factorial arrangement (two sanitary conditions: clean (CL) and unclean (UCL), and five dietary treatments (SID Trp : Lys (16, 18, 20, 22 and 24%)). In each sanitary condition, blood was collected on days 0 and 14 to determine plasma urea nitrogen and on day 14, ileal tissue (one pig per pen) was collected for the measurement of gut morphology. Pigs kept under UCL conditions had lower growth rate (P < 0.05) than under CL conditions. Under CL conditions, the estimated optimal SID Trp : Lys for average daily gain (ADG) was 19.7% whereas under UCL conditions these values were 20.5% and 19.0% for ADG and gain-to-feed ratio, respectively. Under CL conditions, increasing SID Trp : Lys reduced (linear, P = 0.05) plasma urea nitrogen concentration but had no effect (P > 0.10) on villous height (VH), crypt depth ( CD) and VH : CD. In conclusion, an SID Trp : Lys to optimize ADG for pigs raised under UCL conditions was higher (4%) than CL conditions.