Measurement of muscle size is often used to estimate muscle strength. The validity of these measurements as indicators of muscle strength is unclear however. This study investigated the relationship of two simple measures of muscle size (tape and ultrasound) with the concentric isokinetic quadriceps and hamstrings strength of a group of Irish Gaelic footballers (n = 25). Both tape and ultrasound demonstrated moderate to excellent reliability at measuring quadriceps and hamstrings muscle size (ICC's 0.69-0.99). Hamstrings were significantly stronger on the dominant limb at 60 degrees/sec (p = 0.046) and 180 degrees/sec (p = 0.005), but not at 300 degrees/sec (p = 0.092). There was no significant difference in quadriceps strength at any speed (all p > 0.05). Dominant limbs were significantly larger using the tape measure (10 cm level: p = 0.005, mid-thigh level: p = 0.003). The dominant hamstrings (p < 0.001), but not the dominant quadriceps (p = 0.399), were significantly larger on ultrasound. There were statistically significant correlations between muscle strength and muscle size measurements, especially for the tape measurements. However, despite the fact that the dominant limb muscles were both stronger and larger, the strength of these correlations was only weak to moderate (r = 0.176-0.586). The results suggest that both tape measurement and ultrasound, while reliable, are of limited use as indicators of thigh muscle strength.