To determine the prevalence of psychopathology among those attending a primary care centre and discover its diagnostic distribution using the criteria of the DSM-III-R. A crossover study of the prevalence of psychopathology. A two-phase study was performed by systematic sampling of one of every four visits. In the first phase the GHQ-28 was used. For those with suspected psychopathology (cut-off point 5/6) a semi-structured interview, Goldberg's CIS, was performed. They were assigned a diagnosis from the DSM-III-R. Maluquer Salvador Health Centre, Girona. A sample of 400 patients between 15 and 65 who attended the Centre for any reason. There were only 2.9% losses (2.1% in the first phase and 0.8% in the second). Overall prevalence of psychopathology was 38.8% (Cl 34.05-43.55), with depression accounting for 10.3% (major depression 6% and dysthymia 4.3%), anxiety 13.8% (generalised 7.3%, panic 3% and obsessive/compulsive 3%) and adaptive disorder 9.5%. One of every three patients attending a health centre suffers from some kind of psychiatric disorder.