Neurobiology. In the article “Fluctuations and stimulus-induced changes in blood flow observed in individual capillaries in layers 2 through 4 of rat neocortex” by David Kleinfeld, Partha P. Mitra, Fritjof Helmchen, and Winfried Denk, which appeared in number 26, December 22, 1998, of Proc. Natl. Acad. Sci. USA(95, 15741–15746), the authors request that the following correction be noted. Two-photon laser scanning microscopy was used to measure the flow of red blood cells in capillaries in the living brain. In this manuscript, we also reported the length-constant for attenuation of the signal (i.e., two-photon excited fluorescence) as a function of penetration depth into the cortex. As a result of an error in our analysis, the length-constants for each data set were overestimated. The correct value for the data of Fig. 1 is 100 μm (instead of 140 μm), and the range of values over all data sets is from 100 μm to 170 μm (instead of 120 μm to 220 μm). We apologize for any inconvenience caused by our error. None of the other results or conclusions in the original manuscript are affected.