Background. Most patients who develop Perthes disease have unilateral hip involvement. Bilateral involvement occurs in approximately 8-24% of cases. The purpose of this study was to compare the development and outcome of bilateral disease in two groups of patients, with synchronous and metachronous presentation. Material and methods. We reviewed the records and radiographs of 50 patients (41 boys, 9 girls) with bilateral Perthes disease treated conservatively or surgically. The average age at the onset of symptoms was 5 years 10 months (range from 2 years 6 months to 10 years 4 months). The average follow up was 14 years 7 months (range from 7 to 34 years). Results and Conclusions. Synchronous and metachronous involvement of both hips was noted in 52% and 48% of patients respectively. In the second affected hip, Perthes disease developed an average of 14 months later (range from 2 to 47 months). There was significantly more severe necrosis according to the Herring classification and worse outcome according to the Stulberg classification on the second side. In general, Stulberg class appeared not to correlate with synchronous or metachronous presentation. In the whole group with bilateral Perthes disease, better outcome was noted among younger patients and in hips with less severe avascular necrosis. Physicians should not preclude the diagnosis of Perthes disease in those children who present with both hips in the same Reinberg stage of the disease, because 19 out of 26 of our patients with metachronous onset presented with exactly this picture.