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SmMn2Ge2 shows reentrant ferromagnetism. Using flux grown single crystals we find that on cooling through the Curie temperature (Tt1=350 K) the material becomes ferromagnetic (FM). Upon further cooling, at Tt2=150 K an antiferromagnetic (AFM) phase is reached which is stable until Tt3=100 K when there is a transition to FM. The transitions at Tt1 and Tt2 are first order with a temperature hysteresis of about 4 K. Using ac susceptibility to determine the various transition temperatures as a function of pressure (P) we find that Tt3 rapidly decreases with increasing P and at relatively low pressures falls below our measuring range (90 K). Tt2 increases nearly linearly with P and reaches 330 K and 11 kbar. Tt1, drops slowly with increasing P and at 11 kbar the high temperature FM region ends. An x-ray determination of the lattice parameters (at atm pressure) shows that in the AFM region the a0 lattice parameter is decreased, discontinuously changing at Tt2 and Tt3. The c0 lattice parameter is essentially unchanged going through this transition region. These observations indicate that the magnetic properties of SmMn2Ge2 are very sensitive to interatomic spacing. Measurements with a differential scanning calorimeter show that, on heating, the transition at Tt2 is endothermic.