Strategic IS planning (SISP) is an important area for IS research. This study develops the SISP research agenda by examining, in a single study, how the SISP approach used, the SISP objectives of organisations and key dimensions of the SISP context relate together with SISP success. A conceptual model is developed based on SISP theory, strategy process theory and organisational theory. The proposed model was tested with a mail survey of the IT directors (or equivalent) of 70 UK organisations and structural equation modeling was used to analyse the data. Results support an influence on SISP success from both SISP approach and SISP objectives. Within SISP context only the IS maturity of the organisation and the orientation of its business strategy were found to strongly influence SISP success. No support was found for the influence of external environment, organisational structure or IS function structure on SISP success. SISP objectives were also found to influence SISP success indirectly through SISP approach. Similarly IS maturity and strategy orientation influence SISP success indirectly through SISP approach. These findings contribute to SISP theory and potentially provide an improved model for practitioners to pursue higher levels of success from their SISP activities.