['Suspected toenail onychomycosis is a frequent problem. Clinical diagnosis has been considered inadequate.', "To assess the diagnostic accuracy of clinical findings for detecting fungi in toenails, and to develop and validate a clinical diagnostic rule aimed at improving dermatologists' diagnosis of onychomycosis.", 'A cross-sectional diagnostic study was performed including a total of 277 patients seen by 12 dermatologists. The gold standard was the presence of dermatophytes on culture or a positive nail plate biopsy. For each sign we described prevalence, sensitivity, specificity, positive and negative predictive values, and likelihood ratios for positive and negative results. We developed a diagnostic clinical rule and validated it in a subsample.', 'Helpful findings to predict the presence of fungi are: previous diagnosis of fungal disease; abnormal plantar desquamation (affecting > 25% of the sole); onychomycosis considered the most probable diagnosis by a dermatologist; and presence of interdigital tinea. When dermatologists considered onychomycosis the most probable diagnosis and plantar desquamation was present (13% of patients), the positive predictive value for presence of fungi was 81%. When both signs were absent (34% of patients), the positive predictive value for absence of fungi was 71%. In other situations, clinical diagnosis might not give enough information to decide on therapy.'] Is clinical diagnosis of toenail onychomycosis possible in some patients : cross-sectional diagnostic study and development of a diagnostic rule?