Background The psychiatric emergency service (PES) is a major hub in the mental health care delivery system. The aim of this study was to more precisely define what psychiatrists consider to be a psychiatric emergency and to examine the underlying basis of this assessment. Methods Over twenty-two thousand PES visits were assessed prospectively for pertinence and urgency by psychiatrists in four functionally and structurally different services in the province of Quebec, Canada. This study took place between July 15 1996 and August 31, 2004. Results Overall, 57% of visits were judged pertinent and urgent (P/U), 30% pertinent but not urgent (P/NU) and 13% neither pertinent nor urgent (NP/NU). Between 50 and 60% of P/U tagged visits were diagnosed with an affective or a psychotic disorder, often with a suicidal content. They also more frequently resulted in a short-term observation in the PES or a hospitalization. Variables suggesting the presence of a behaviorally disturbed state (aggressive behaviors, involuntary or police referrals) were equally likely to be found in P/U or NP/NU visits. Legal confinement following the consultation was almost exclusively seen in visits judged P/U. The percent of visits tagged P/U at the four individual sites varied substantially above and below the 57% value for the combined data. Interestingly, no major inter-site differences in diagnostic profiles for the three pertinence and urgency anchor points were found that might account for this variability. Finally, visits from high frequency users were less likely to be judged P/U than visits from patients attending less frequently. Conclusion Primary consideration for a P/U tag was a visit characterized by a behaviorally disturbed state and/or, suicidal ideation (or attempts) within the context of either an underlying psychotic or affective disorder, especially if poor judgment was an issue. Some specific diagnoses appeared to qualify the above core clinical considerations, increasing or decreasing the probability of a P/U tag. Finally, non-clinical site-specific factors related to the individual services themselves, such as the number of readily available specialized resources, also appeared to qualify this assessment. These data may prove useful for the future development of this service.