POSITIVELY SLOPED I-S curves have recently played a role in the discussion of monetary theory and policy. David Meiselman, for example, has said, My own judgment is that under a wide range of circumstances, the I-S curve is best taken to have a positive slope. [1, p. 147] William Silber, in a recent article in this Journal discussed the effectiveness of monetary policy when there is a positively sloped I-S curve. [2]. The purpose of this note is to point out that, given a Keynesian model, such as is assumed in the Silber article, an upward sloping I-S curve leads to properties that are not consistent with historical experience. The reason for this is simple: Given a model with a positively sloped I-S, stability depends on adventitious circumstances. That is, stability occurs if it is assumed, as Silber does, that the central bank controls the money stock so that money is exogenous. But if there is a positive I-S, instability may occur when the authorities use interest rate targets and do not allow interest rates to rise or fall sufficiently to restore equilibrium, so that deviations from equilibrium may lead to cumulative, unbounded increases or decreases in GNP. Such behavior of GNP has not been observed in periods when the authorities have fixed interest rates, such as in the pre-Accord era when the Federal Reserve pegged the price of government securities, nor does such behavior correspond to the later performance of the economy when the Federal Open Market Committee used interest rate targets. The potential instability of the economy arises with a positively sloped I-S curve since the marginal propensity to spend out of GNP--the marginal propensity to consume plus the marginal propensity to invest-must be greater than one.' Then given fixed interest rates the geometric series representing the investment multiplier never converges. For example, an increase in government spending by $1 raises spending by more than $1, which, in turn, raises spending and income by successively greater amounts. Over time, the rise in income increases without bounds. The comparative statics multiplier, showing the change in income that is required to restore equilibrium, also reflects the peculiar properties of the model under these circumstances. In contrast to its usual character, the multiplier is negative so that for an increase in autonomous spending the income required for equilibrium is lowered. This is illustrated in Figure 1, where the I-S curve is shifted upward due to an increase in autonomous spending; in this case, at the same level of income, a higher level of interest rates is required to create no excess demand for current output.