A multicentre trial for the treatment of dermatophyte onychomycosis of the toenails with terbinafine was carried out in Australia and New Zealand. Between eight and 12 nail samples were obtained from each of the 118 patients in the 48-week trial, and each sample was investigated by direct microscopy and culture for dermatophyte and non-dermatophyte fungi. Patients were randomized to treatment with terbinafine at 250 mg/day or placebo for the first 12 weeks of the study, then non-responders were offered a 12-week course of terbinafine from week 28. All patients had a dermatophyte infection. In 42 patients (36%) microscopy and mycological culture identified dermatophytes alone. In the remaining 76 patients (64%), a non-dermatophyte mould or yeast was also isolated at some stage during the trial, but in only three patients did the same non-dermatophyte persist in two or more successive nail specimens. The presence of a fungal contaminant in addition to a dermatophyte had no apparent effect on the efficacy of treatment with terbinafine. We conclude that non-dermatophyte moulds and yeasts are generally found as contaminating organisms in dermatophyte onychomycosis, secondary to the dermatophytes, and that they do not influence the outcome of treatment.