This study was conducted on 386 patients (341 boys and 45 girls) with Perthes' disease to evaluate the characteristics of bilateral Perthes' disease. We experienced 34 bilateral cases (8.8%) among these patients them. There were no significant differences between boys (8.8%) and girls (8.9%). The average age at onset was significantly lower in bilateral cases (4.62 y. o.) than in unilateral cases (6.43 y. o.). The average interval period between earlier and later onsets in bilateral cases was 15.7 months. In 30 cases (88%), later onset occurred within 24 months. In four cases, Perthes' disease occurred at both sides at the same time. There were no significant differences in the severity of involvement between unilateral and bilateral cases, and between earlier and later onsets in bilateral cases.