Background: The 5 main types of onychomycosis are distal and lateral subungual onychomycosis (DLSO), white superficial onychomycosis (WSO), proximal subungual onychomycosis (PSO), Candida onychomycosis, and total dystrophic onychomycosis (TDO) (primary or secondary type). In the literature there is infrequent discussion about 2 types of onychomycosis present in the toenails of the same individual. Objective: We attempted to determine the prevalence and etiologic organisms of DLSO and WSO occurring in the same individual. Methods: We surveyed 4411 subjects presenting to dermatology offices for causes other than the management of onychomycosis. In each patient the toenails were examined. If they appeared abnormal, nail material was obtained for mycologic evaluation; partitioned sampling was performed when more than one type of onychomycosis was present. Results: In our series, 39 (0.9%) of 4411 patients had the combination DLSO and WSO, compared with 417 (9.4%) and 111 (2.5%) who had DLSO and WSO, respectively. After controlling for age and sex in the general population, the projected prevalence rates of DLSO, WSO, and combined DLSO and WSO in the province of Ontario, Canada were 7.1%, 1.5%, and 0.5%, respectively. The combination of DLSO and WSO in the toenails of an individual occurred more frequently than that predicted by chance alone (P < .0001). Nine (0.2%) of 4411 subjects had DLSO and WSO on the same nail. In 23 (59%) of 39 subjects both the DLSO and WSO were associated with Trichophyton mentagrophytes . In the remaining 16 subjects other organisms cultured were T rubrum , Acremonium spp, Aspergillus spp, Fusarium oxysporum , and Onychocola canadensis . In 33 (84.6%) of 39 subjects with the combination of DLSO and WSO on the toenails, the same fungal organism was associated with both the DLSO and WSO. Conclusion: When both DLSO and WSO are concurrently present in the toenails of an individual, partitioned sampling (ie, sampling for each of the two types of onychomycosis) may provide us with a better understanding of the different organisms associated with the onychomycosis and the relationship between the two types of onychomycosis. (J Am Acad Dermatol 1999;41:938-44.)