The triple system HD 158926 (λ Sco) has been observed interferometrically with the Sydney University Stellar Interferometer, and the elements of the wide orbit have been determined. These are significantly more accurate than the previous elements found spectroscopically. The inclination of the wide orbit is consistent with the inclination previously found for the orbit of the close companion. The wide orbit al so has low eccentricity, suggesting that the three stars were formed at the same time. The brightness ratio between the two B stars was also measured at λ = 442 and 700 nm. The brightness ratio and colour index are consistent with the previous classification of A. Sco A as B1.5 and λ Sco B as B2. Evolutionary models show that the two stars lie on the main sequence. Since they have the same age and luminosity class (IV), the mass-luminosity relation can be used to determine the mass ratio of the two stars: MB/MA = 0.76 ±0.04. The spectroscopic data have been reanalyzed using the interferometric values for P, T, e and ω, leading to revised values for ai sin i and the mass function. The individual masses can be found from the mass ratio, the mass function, spectrum synthesis and the requirement that the age of both components must be the same: MA = 10.4 ±1.3 and MB = 8.1 ±1.0 M⊙. The masses, angular semimajor axis and the period of the system can be used to determine the dynamical parallax. We find the distance to λ Sco to be 112 ±5 pc, which is approximately a factor of 2 closer than the Hipparcos value of 216 ± 42 pc.