The purpose of this study was to determine whether frequencies of any current primary or secondary (organic) psychiatric diagnoses in consultation-liaison (C-L) patients are associated with patient report of past history of mental health or substance use disorder treatment. Clinical information recorded in 134 C-L patients was retrospectively analyzed with the chi-square test, two-tailed, or Fisher's Exact Test when the sample sizes were less than or equal to 5. The results revealed that the overall likelihood of the existence of a secondary psychiatric disorder diagnosis is elevated when there is no treatment history (P = 0.009). A primary psychiatric disorder is more likely to be diagnosed when there is treatment history (P = 0.009). Specific psychiatric diagnoses more likely to be present when there is no treatment history are organic mood disorder, depressed (P = 0.020) and adjustment disorder (P = 0.010). Specific diagnoses more likely to be present when there is treatment history are psychoactive substance-withdrawal syndrome (P = 0.014) and alcohol abuse or dependence (P = 0.028). The authors conclude that psychiatric treatment history can help C-L psychiatrists with differential diagnostic considerations.