INTRODUCTION Despite intensive investigations the original of a monarthritis may remain unclear. In this present study, relevant parameters for prognosis are sought for such cases. METHODS In a retrospective analysis 501 records, kept over a period of thirty years, of patients with a first diagnosis of arthritis have been evaluated. RESULTS The percentage of monarticular arthritis which remained unclear is 48% (n = 246). Of these patients, 63% (n = 318) were male, 37% female. A diagnosis was possible in 26% by puncture, in 47% by biopsy of the synovia. White blood cell count and sedimentation rate were of no help for diagnosis. The great joints were mostly affected, especially the knee (148 cases) and the hip (115 cases). Diagnosis of monarthritis was seldom in younger patients (7%). 119 of the 246 cases of arthritis of unknown origin became pain free (48.4%). Influence of age or sex could not be shown in this group of patients. However, younger patients tended more often to become pain free (73% in the group under 10 years of age). DISCUSSION Exact history, precise primary examination, laboratory tests and radiological investigations are most important for diagnostic reliability. Whenever possible a puncture of the joint and in persisting cases also a biopsy should be performed. With careful examination more cases should be diagnosed.