Early onset forms of bipolar disorder may be difficult to distinguish from schizophrenia. Although operational criteria have become more precise, and there are more diagnostic systems to catalogue a psychotic adolescent, the clinicians continue having difficulties.

To know the stability of the diagnosis of definitively bipolar patients with psychotic symptoms during the episodes, and factors that can influence other psychotic diagnoses in the first episode of the illness.

140 bipolar patients of Araba, (Basque country) were included during 2 years. Patients were divided into two groups: unstable diagnoses (UD)(bipolar patients with an initial diagnosis of other psychosis) and stable diagnoses (SD) of bipolar disorder. Clinical and sociodemographic data were obtained.

The mean age at onset was significantly lower in the UD group (p = 0.004). It was rare to have an unstable diagnosis when the first psychotic episode occurred after age 38 (p = 0.008). There were more singles in the UD group (p = 0.010).

The presence of mood incongruent psychotic symptoms was more frequent in the UD group (p < 0.001).

Mood incongruent psychotic symptoms variable was the only independent factor significantly associated with an unstable diagnosis in the multivariate analysis (p = 0.036).

One third of the patients have been previously diagnosed with other psychotic illness.

This study suggests that the most important factor is the clinical picture, especially the presence of mood-incongruent psychotic symptoms. A correct diagnosis is important because patients who are prescribed mood stabilizers have lower rates of rehospitalisation than those who are not treated with mood stabilizers.