BACKGROUND: The aim of this comparative study was to investigate the type and frequency of diagnoses preceding adult bipolar disorder (BD) and schizophrenia (SZ). METHODS: A follow-back study of all preceding diagnoses in all patients aged 21-34 years with a primary, first time diagnosis of BD (N = 784) or SZ (N = 1667) in 2008 to 2010. Data were taken from the Danish Psychiatric Central Research Register (DPCRR) including ICD-10 and ICD-8 diagnoses. RESULTS: The numbers of patients with any preceding diagnoses amounted to 69.3% in BD and 76.6% in SZ with affective disorders (excluding BD) being the most frequent preceding diagnosis (46.6 vs. 28.0%), followed by psychoses (PSY) other than SZ (14.2 vs. 41.5%, p < .001), and substance use disorders (SUD) (16.1 vs. 26.9%, p < .001). Reactions to severe stress were equally frequent in both samples (26.3 vs. 26.6%) as were personality disorders (21.8 vs. 22.4%) and ADHD (4.2 vs. 3.5%), whereas rates of conduct disorders (1.7 vs. 3.1%) were rather low in both samples. Very few of the preceding diagnoses had their onset in childhood and adolescence. Overall patients with SZ had a minor but statistically significant earlier onset of any psychiatric disorder compared to BD (mean age: 23.3 vs. 22.5, p < .001). Regression analyses indicated that BD was associated with an increased risk of having experienced preceding affective disorders and ADHD, while SZ was associated with an increased risk of preceding substance use disorders, psychosis, anxiety disorders, and personality disorders. CONCLUSIONS: Specific developmental trajectories of preceding disorders were delineated for BD and SZ with affective disorders being more specific for BD and both SUD and PSY more specific to SZ. There are different patterns of vulnerability in terms of preceding diagnosis in young adults with BD and SZ.