  The HyperCP collaboration has recently reported the observation of three
events for the decay Sigma^+ -> p mu^+ mu^-. They have suggested that new
physics may be required to understand the implied decay rate and the observed
M_{mumu} distribution. Motivated by this result, we re-examine this mode within
the standard model, considering both the short-distance and long-distance
contributions. The long-distance part depends on four complex form-factors. We
determine their imaginary parts from unitarity, fix two of the real parts from
the Sigma^+ -> p gamma measurements, and estimate the other two with
vector-meson-dominance models. Taking into account constraints from Sigma^+ ->
p e^+ e^-, we find that Sigma^+ -> p mu^+ mu^- is long-distance dominated and
its rate falls within the range suggested by the HyperCP measurement.
