We consider the nonlinear heat equation ut − ∆u = |u| p + b|∇u| q in (0, ∞) × R n , where n ≥ 1, p > 1, q ≥ 1 and b > 0. First, we focus our attention on positive solutions and obtain an optimal Fujita-type result: any positive solution blows up in finite time if p ≤ 1 + 2 n or q ≤ 1 + 1 n+1 , while global classical positive solutions exist for suitably small initial data when p > 1 + 2 n and q > 1 + 1 n+1 . Although finite time blow-up cannot be produced by the gradient term alone and should be considered as an effect of the source term |u| p , this result shows that the gradient term induces an interesting phenomenon of discontinuity of the critical Fujita exponent, jumping from p = 1 + 2 n to p = ∞ as q reaches the value 1 + 1 n+1 from above. Next, we investigate the case of sign-changing solutions and show that if p ≤ 1 + 2 n or 0 < (q − 1)(np − 1) ≤ 1, then the solution blows up in finite time for any nontrivial initial data with nonnegative mean. Finally, a Fujita-type result, with a different critical exponent, is obtained for sign-changing solutions to the inhomogeneous version of this problem.2010 Mathematics Subject Classification. 35K05; 35B44; 35B33. Local well-posedness remains true under weaker regularity assumptions on the initial data; however we shall not discuss this here, since our focus is on the large time behavior of solutions.