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We have studied the role of long-range interactions on the thermodynamics of magnetic systems. We have simulated, through the Monte Carlo method, magnetization curves of a two-dimensional classical Ising model including a long-range dipole-dipole-like interaction, where the range of interaction is tuned by a parameter ␣. Based on the conjectures of Tsallis statistics, we show that, for ␣/dр1 (dϭ2), the appropriate form of the equation of state is given by M /Nϭm(T*,H*) with T*ϵT/N* and H*ϵH/N*. The normalization factor N*͓N*ϵ(N (1Ϫ␣/d) Ϫ1)/(1Ϫ␣/d)͔ emerges from the nonextensivity of thermodynamic variables of energy type. The crossover from nonextensive to extensive behavior at ␣/dϭ1 occurs smoothly and similarly to other quite different systems, thus suggesting it to be a general result. ͓S0163-1829͑97͒06009-8͔ PHYSICAL REVIEW B

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