The influence of the degree of transmitting beam coherence on the bit error rate in free-space optical communication systems is studied. It is found that there are optimal values of the output power and degree of coherence of a transmitter optical beam, defined by the Fried radius, for all types of propagation paths (horizontal, vertical, or inclined) and different degrees of manifestation of turbulent effects. The optimal degree of coherence can be determined from the calculated BER minimum.