A study was carried out to determine whether the diagnosis of non-specific urethritis was affected by differences in the microscopical interpretation of urethral smears between individual observers (interobserver variation) and the same observer on separate occasions (intraobserver variation). A marked degree of both intraobserver and interobserver variation was found which--depending on the diagnostic criteria adopted--could affect both the diagnosis and treatment of many patients attending a clinic of genitourinary medicine.