The purpose of this study was to compare compliance with recommendations and clinical outcomes between formal and informal infectious disease specialist consultations. Six hundred twenty-seven consecutive adult inpatients who received an infectious disease consultation in a university-affiliated hospital were included. After adjusting for quintile of propensity score, we compared compliance with the consultant's recommendations and clinical outcomes for 443 (70.7%) and 184 (29.3%) formal and informal consultations. Informal and formal consultations were associated with comparable levels of compliance with recommendations for antimicrobial treatment (86.5% vs 88.9%; adjusted odds ratio [aOR], 0.63; 95% confidence interval, 0.34-1.14; = 0.13) and diagnostic or monitoring tests (72.6% vs 72.0%; aOR, 0.91 [0.53-1.57]; = 0.73). The rates of early clinical improvement (58.2% vs 58.6%; aOR, 1.11 [0.70-1.74]; = 0.66), subsequent consultation (34.2% vs 36.3%; aOR, 0.80 [0.53-1.21]; = 0.29), in-hospital mortality (4.9% vs 8.4%; aOR, 0.55 [0.24-1.24]; = 0.15), and the median length of stay (23 vs 20 days; aOR of discharge, 0.90 [0.74-1.10]; = 0.30) did not differ depending on the type of consultation. This study provides observational evidence that informal consultations result in levels of compliance with recommendations comparable to formal consultations, without compromising patient safety. Further study is needed to refine the criteria for requesting or providing informal rather than formal consultations.