Frequent attenders account for a large proportion of primary care (PC) contacts, referrals, and prescriptions. Psychosocial and emotional distress is related to the high use of health services. Few studies have focused on the association between mental disorders assessed using structured interviews and frequent use of PC services.The aim of this study was to determine the factors associated with frequent attendance at primary healthcare units, focusing specifically on mental disorders. A two-phase screening epidemiological study comparing frequent attenders and routine attenders in five primary health care units was designed. Three hundred eighteen frequent attenders and 203 patients who attended the same units on a routine basis were compared. Sociodemographic and clinical data were obtained from statistical records and medical charts. Patients with a total score equal or higher than 7 points on the General Health Questionnaire-28 (GHQ-28) were interviewed using the Schedules for Clinical Assessment in Neuropsychiatry. All the scores obtained on the GHQ were statistically different in the two populations. Frequency of mental disorders also differed significantly between both groups, with somatoform and affective disorders being the most prevalent ICD-10 categories among frequent attenders. The presence of depressive disorders and somatoform disorders is the most powerful predictive factor for frequent attendance. High comorbidity was found among frequent attenders with somatoform disorder. Frequent attendance at primary healthcare units is associated with depressive and somatoform disorders. Psychiatric comorbidity could be a confounder, particularly because affective and somatoform disorders often overlap in PC patients.