Modifications of the Physiologic Stability Index (PSI) and Therapeutic Intervention Scoring System (TISS) were used to evaluate the physiologic stability and need for therapeutic intervention in 55 infants hospitalized in the newborn ICU. After modifying the PSI to reflect neonatal physiology, we found that PSI scores correlated significantly with TISS values (r = .75, p < .001) and Nursing Utilization Management Intervention System (NUMIS) classifications (r = .62, p < .001). TISS values also correlated with NUMIS scores (r = .72, p < .001). PSI and TISS scores increased significantly with each increase in NUMIS classification (p < .001 for all comparisons). PSI and TISS scores decreased significantly between admission and either discharge (n = 41) or day 14 of hospitalization (n = 14, p < .001). PSI and TISS scores were greater on days 1 and 5 in infants with hyaline membrane disease when compared with infants with transient tachypnea (p < .001). Infants with PSI scores ≥4 and TISS scores ≥7 on day 1 took significantly longer to achieve adequate protein-calorie intakes than infants with lower first-day scores (p < .002). The modified PSI and the TISS scoring systems are both useful objective measurements of the degree of physiologic instability in newborn infants requiring intensive care, and both scores identify those at increased risk for nutritional morbidity.