This paper studies the existence of a positive solution to the second-order periodic boundary value problemwhere the nonlinear term f (t, u) may be singular at t = 0, t = 2π and u = 0. When there exist the limit functions lim u→+0 f (t, u)/u and lim u→+∞ f (t, u)/u, we prove that the problem has a positive solution provided that the integrations of the limit functions are appropriate.