In 1985 hydraulic fracturing rock stress measurements were conducted down to a depth of 500 m in a vertical borehole at Lavia in Finland. Because of the large number of horizontal hydrofractures encountered, the in-situ stress determination obtained by using the classical theory of Hubbert and Willis [1] gave poor results. This paper presents an alternative theory for the determination of in situ stresses from hydraulic fracturing tests when the hydrofractures are either vertical or horizontal. The theory uses the Hoek and Brown criterion [2] instead of the maximum tensile stress failure criterion. For horizontal fractures, the theory can be used to determine the difference in magnitude between the horizontal stress components and their domains of variation. In addition to the breakdown and shut-in pressures, obtained from the hydraulic fracturing tests, the theory requires knowledge of the unconfined compressive strength, the Poisson's ratio of the rock and one empirical constant, m, associated with the Hoek and Brown failure criterion or, alternatively, the tensile strength of the rock. The new theory is used to analyze the data from hydraulic fracturing tests at Lavia, Finland, where both vertical and horizontal hydrofractures were observed. This paper demonstrates that important information on in situ stresses may be obtained even though a large number of horizontal hydrofractures have been created. Although the absolute stress magnitudes cannot be determined, a much better understanding of the stress rate is obtained. Analysis of field data shows that the state of stress at Lavia is characterized by high horizontal stresses, with a magnitude of about 50 MPa for the maximum horizontal stress at 500 m depth. The results also show large differences in magnitude between the major and minor horizontal stresses, this difference being of the order of 20–25 MPa.