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It has been proposed that the arrived photon count obeys negative binomial (NB) distribution when a thermal optical source is used. In this letter, we have analyzed the bit-error rate (BER) of an optical spectral-amplitude-coding CDMA system using NB distribution. It has been shown that when the number of simultaneous users is small, the resultant BER is much lower than that calculated using Gaussian approximation. When the number of simultaneous users is large (for example N>19 when the effective power is -27 dBm), both NE distribution and Gaussian approximation give almost the same value for the BER of the system.