This is a retrospective study of the agents, clinical aspects, sources of infection and therapy of onychomycosis in children. In the period 1989–2000, we observed 46 consecutive children, until 16 years of age with onychomycosis (29 boys, 17 girls, mean age 10.8 years). Dermatophytes were isolated in 30 cases (Trichophyton rubrum in 22 cases, Trichophyton mentagrophytes in five, Epidermophyton floccosum in two and Trichophyton violaceum in one) and Candida spp. in 16, associated with Trichophyton rubrum in two. Moulds were isolated in three children (Fusarium oxysporum in one, Scopulariopsis brevicaulis in another and Aspergillus fumigatus associated with Trichophyton rubrum in a third). The commonest features were distal and distolateral subungual hyperkeratosis in dermatophyte infections (93%) and onychodystrophy and paronychia in Candida infections (56% and 50% respectively). Forty patients achieved clinical and mycological recovery. It is appropriate to suspect onychomycosis in children, perform microbiological diagnosis and undertake early treatment. An approach of this kind may help to prevent nail dystrophy and the spread of infection.