Idiopathic nephrotic syndrome is the most common form of nephrotic syndrome in children, but little is known about the etiology of the disease. To obtain new insights into the etiology of the disease, we studied circannual patterns of initial episodes and relapses of steroid-sensitive nephrotic syndrome (SSNS). From 1986 to 2003, there were 45 children with SSNS in our hospital, and they experienced 43 relapses during that period. Initial episodes of SSNS were found to show significant circannual variation with an autumn peak both by Roger's test and Freedman's test, and the circannual pattern was more obvious in patients with high serum IgE levels. Chronological evaluation by means of Fourier analysis showed a clear circannual pattern. In contrast, relapses of SSNS showed circannual variation with a spring peak, which was a result of a high frequency of relapses after upper respiratory infections in January. These results suggest that circannual variation in initial episodes of SSNS might be associated with allergic predisposition, whereas circannual variation in relapses might be associated with preceding upper respiratory infections.