BACKGROUND:Nondermatophyte moulds (NDM) may be found as aetiological agents or as contaminants in onychomycosis. The classic and most used criteria for the diagnosis of NDM are those established by English in 1976. OBJECTIVES:The aim of this article is to re-evaluate the laboratory criteria for the diagnosis of NDM in onychomycosis. PATIENTS AND METHODS:Patients with suspected NDM of the nail underwent five consecutive examinations by both KOH and mycological culture; at the first visit, three samples from the affected nail were taken and were examined separately. Later those patients underwent four consecutive examinations; during this stage only a single sample for both KOH and culture was taken. We compared the culture results obtained from the three nail samples obtained at the first visit with the results from the four consecutive visits. RESULTS:We noted a clear trend showing that as the number of positive cultures increases (one to three cultures) during the first examination, the percentage of subsequent positive cultures, taken during the four consecutive visits, also increased. CONCLUSIONS:We suggest that when NDM infection is found in the first culture, the patient should be re-examined in a subsequent visit in which three separate samples are taken from the affected nail. If NDM is confirmed in all three cultures, the diagnosis of NDM is established. Treatment should be recommended in patients who show positive results in all three cultures.