Patients with acute respiratory failure (ARF) show changes in skeletal muscle structure and strength. The purpose of this study was to examine the relationship between muscle thickness, echogenicity, and strength in patients with ARF. Methods: Thirteen (6 females, 7 males) patients with ARF participated in the study. Knee extensor strength was measured via a handheld dynamometer and quadriceps images were obtained via ultrasonography at hospital discharge. The ultrasonograms were used to obtain muscle thickness and both mean ± SD echogenicity of the rectus femoris, vastus lateralis, and vastus medialis. Partial correlations, controlling for age, body mass index, and fluid intake were used to describe the relationships among knee extensor strength and echogenicity and muscle thickness. Results: Knee extensor strength and vastus lateralis SD echogenicity were significantly correlated when controlling for age, body mass index, and fluid intake (r = 0.69, P =.029). Knee extensor strength and vastus lateralis echogenicity were 19.1 ± 8.0 kg and 19.8 ± 5.4 units, respectively. No other correlations between strength and ultrasound measures were found to be significant. Conclusion: These results show skeletal muscle echogenicity to be significantly correlated with skeletal muscle strength in patients with ARF. As such, it may be useful in identifying muscle weakness in these patients when they are unable or unwilling to perform voluntary strength testing.