We categorized subjects into 3 duration groups of olfactory loss: group A, less than 24 months; group B, 24-48 months; and group C, longer than 48 months. The presence of anosmia increased with duration of olfactory loss. Specifically, most of the patients with the longest duration of olfactory loss were anosmic (group C, 16 hyposmic vs 34 anosmic; χ2 = 6.48; P = .01); no significant differences were found between the frequencies of hyposmia and anosmia in group A (55 hyposmic vs 53 anosmic) or B (25 hyposmic vs 21 anosmic). In addition, the percentage of patients with different causes of olfactory disorders was associated with the duration of the disease. That is, URI olfactory disorders were most frequent in duration groups A (56%) and B (57%) and least frequent in group C (26%). The percentage of SND increased with increasing duration (group A, 5%; B, 9%; and C, 24%); similar findings were made for idiopathic olfactory loss (group A, 14%; B, 4%; and C, 30%) but not for trauma (group A, 22%; B, 26%; and C, 16%) or other causes (group A, 4%; B, 4%; and C, 4%). Accordingly, statistics revealed that the percentages of the 4 most frequent causes were significantly different when the duration of disease was shorter than 48 months (group A, χ2 = 66.2 with P<.001; group B, χ2 = 32.3 with P<.001), but not when the duration of disease was longer than 48 months.