They found that "significant risk factors for invasive penile cancer:
lack of circumcision during childhood, phimosis and cigarette smoking."

More:

"Men not circumcised during childhood were at increased risk of
invasive (penile cancer) (OR = 2.3, 95% CI 1.3-4.1)."

"Among men not circumcised in childhood, phimosis was strongly
associated with development of invasive penile cancer (OR = 11.4, 95%
CI 5.0-25.9)."

And from the main body of the study itself:

"We posit that men who have never been circumcised may be at
higher risk of HPV acquisition and persistence even though they
appear to have engaged in lower-risk sexual activity than men
who are circumcised in childhood."

==========================
Penile cancer: importance of circumcision, human papillomavirus and
smoking in in situ and invasive disease.

Daling JR, Madeleine MM, Johnson LG, Schwartz SM, Shera KA, Wurscher
MA, Carter JJ, Porter PL, Galloway DA, McDougall JK, Krieger JN.

Program in Epidemiology, Division of Public Health Sciences, Fred
Hutchinson Cancer Research Center, Seattle, WA 98109-1024, USA.
jdaling@fhcrc.org

Few population-based case-control studies have assessed etiologic
factors for penile cancer. Past infection with high-risk human
papillomavirus (HPV) is a known risk factor for penile cancer;
however, few previous studies have related the HPV DNA status of the
tumor to potential demographic and behavioral risk factors for the
disease or evaluated whether in situ and invasive penile cancer share
risk factors. Little information is available on the role and timing
of circumcision in the etiology of penile cancer. We conducted a
population-based case-control study in western Washington state that
included 137 men diagnosed with in situ (n = 75) or invasive (n = 62)
penile cancer between January 1, 1979, and December 31, 1998, and 671
control men identified through random digit dialing. Cases and
controls were interviewed in person and provided peripheral blood
samples. Case and control blood samples were tested for antibodies to
HPV16 and HSV-2, and tumor specimens from cases were tested for HPV
DNA. Men not circumcised during childhood were at increased risk of
invasive (OR = 2.3, 95% CI 1.3-4.1) but not in situ (OR = 1.1, 95% CI
0.6-1.8) penile cancer. Approximately 35% of men with penile cancer
who had not been circumcised in childhood reported a history of
phimosis compared to 7.6% of controls (OR = 7.4, 95% CI 3.7-15.0).
Penile conditions such as tear, rash and injury were associated with
increased risk of disease. Among men not circumcised in childhood,
phimosis was strongly associated with development of invasive penile
cancer (OR = 11.4, 95% CI 5.0-25.9). When we restricted our analysis
to men who did not have phimosis, the risk of invasive penile cancer
associated with not having been circumcised in childhood was not
elevated (OR = 0.5, 95% CI 0.1-2.5). Cigarette smoking was associated
with a 4.5-fold risk (95% CI 2.0-10.1) of invasive penile cancer. HPV
DNA was detected in 79.8% of tumor specimens, and 69.1% of tumors were
HPV16-positive. The proportion of HPV DNA-positive tumors did not vary
by any risk factors evaluated. Many risk factors were common for both
in situ and invasive disease. However, 3 factors that did not increase
the risk for in situ cancer proved significant risk factors for
invasive penile cancer: lack of circumcision during childhood,
phimosis and cigarette smoking. The high percentage of HPV DNA-
positive tumors in our study is consistent with a strong association
between HPV infection and the development of penile cancer regardless
of circumcision status. Circumcision in early childhood may help
prevent penile cancer by eliminating phimosis, a significant risk
factor for the disease. (c) 2005 Wiley-Liss, Inc.

http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?
cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=15825185&query_hl=3