The risk of contra-lateral slip in patients with primary unilateral slipped capital femoral epiphysis (SCFE) is difficult to determine. The material consists of 115 patients operated on because of unilateral SCFE between 1968 – 1991. There were 75 boys (65%) and 40 girls (35%). The mean age at the diagnosis was 12.8 years.

Methods. Measurements of such radiological parameters of hip joints as: neck-shaft angle, Alsberg angle, slip angle and Klein’s sign were done. All these measurements were done in three periods. First – at the time of admission, second – at the time of suspected contra-lateral slip and third – at follow up (minimum 2 yrs after subcapital growth plate closure). The mean follow up was 11 yrs (2 – 29)

Results. Contra-lateral slip developed in 73% of patients with Alsberg angle (capital physeal – femoral shaft angle) less than 61° and only in 43% of patients with this angle bigger than 61°. No correlation between developing of contra-lateral slip and femoral neck – shaft angle, slip angle and negative Klein’s sign was found. A positive Kline’s sign in the “healthy” hip was observed in 37% of patients with unilateral SCFE at the moment of the fist slip and in all of them contra-lateral slip and/or early coxarthrosis developed.

Conclusions. More vertical orientation of the proximal femoral epiphysis can be used as risk factor of the contra-lateral slip in patients with primary unilateral SCFE. Femoral neck – shaft angle, slip angle and negative Kline’s sign in the “healthy” hip have no prognostic value according to the contra-lateral slip. More than 1/3 of the patients with primary unilateral slip developed a symptomatic contra-lateral slip.