We conducted a cross-sectional study to determine the prevalence of windswept hip deformity (WSHD) and hip dislocation (HD), and their relationship to asymmetry of muscle tone. A total of 103 persons with spastic quadriplegia were randomly recruited from two study institutions. Data were gathered from medical records and a standardized physical examination. WSHD was observed in 52%, HD in 25%, tonal asymmetry in 52%, hip subluxation in 63%, and hip surgery in 63%. Multinomial regression analyses showed that only tonal asymmetry was related to WSHD (p = 0.0002), whereas increased age (p = 0.016), severity of spasticity (p = 0.004), and direction of tonal asymmetry (p = 0.001) were associated with adverse hip events (WSHD or HD). Adverse hip events were more prevalent than expected. The side of strongest tone was more often dislocated or held in fixed adduction. A hip rotation range of motion algorithm, constructed in this study, may offer early clues to direction of future hip deformity.