We have studied the interaction of H2O masers in star-forming regions (and through it the enhancement of the observed luminosity) as a physical mechanism for the explanation of the very strong H2O maser sources. For each individual maser we have taken a collisional pump model. Such a model can explain the low and medium luminosity masers but not the very powerful ones. We have carried out detailed numerical calculations for both saturated and unsaturated masers and have derived approximate analytic expressions for the expected brightness temperature from interacting masers. We have found that the interaction of two low or OH masers have not been observed yet, but this could be a result of interstellar scattering.