Let
u
=
u
(
x
,
t
)
u = u(x,t)
be a continuous weak solution of the porous medium equation in
R
d
×
(
0
,
T
)
{{\mathbf {R}}^d} \times (0,T)
for some
T
>
0
T > 0
. We show that corresponding to
u
u
there is a unique nonnegative Borel measure
ρ
\rho
on
R
d
{{\mathbf {R}}^d}
which is the initial trace of
u
u
. Moreover, we show that the initial trace
ρ
\rho
must belong to a certain growth class. Roughly speaking, this growth restriction shows that there are no solutions of the porous medium equation whose pressure grows, on average, more rapidly then
|
x
|
2
|x{|^2}
as
|
x
|
→
∞
|x| \to \infty
.