['The relation between knowledge structure and diagnostic performance is unclear. Similarly, variables affecting knowledge structure are poorly understood.', 'The 1st objective was to examine the relation between concepts in knowledge structure and diagnostic performance. The 2nd objective was to examine the relation between the use of diagnostic schemes by small-group preceptors and knowledge structure of medical students.', 'This was a cross-sectional study of 1st-year medical students in 4 clinical presentations: hyponatremia, hyperkalemia, metabolic acidosis, and metabolic alkalosis. The 1st dependent variable was diagnostic success with the number of expert-type concepts in knowledge structure (determined by concept sorting), diagnostic scheme use by preceptors, and clinical presentation as independent variables. The 2nd dependent variable was the number of expert-type concepts in knowledge structure with diagnostic scheme use by preceptors and clinical presentation as independent variables. Data were analyzed using multiple logistic and linear regression.', 'Thirty 1st-year medical students participated. After adjusting for clinical presentation and scheme use by preceptors, the number of expert-type concepts in knowledge structure was associated with increased odds of diagnostic success (odds ratio 1.18 [1.03, 1.35], p = .016). After adjustment for clinical presentation, scheme use by preceptors was associated with increased number of expert-type concepts in knowledge structure (2.22 vs. 1.86, p = .01, d = 0.23).'] Is expert-type knowledge structure in medical students associated with increased odds of diagnostic success?