Effects of sunscreen on skin cancer and photoaging -
"This review of results of randomised controlled trials shows that the evidence, though limited, supports beneficial effects of sunscreen application on the occurrence of skin cancers and skin photoaging"
http://www.ncbi.nlm.nih.gov/pubmed/24417448

http://www.ncbi.nlm.nih.gov/pubmed/7790106
"After adjusting for age, sex, hair colour and holiday weeks spent each year in sunny resorts, the melanoma risk was of 1.50 (95% Cl:1.09-2.06) for regular sunscreens, and of 2.28 (95% Cl: 1.28-4.04) for psoralen sunscreens."

Is the use of sunscreens a risk factor for malignant melanoma?
" The use of sunscreens was not found to protect against developing malignant melanoma. Instead, an unexpected relation between the use of sunscreens and the risk of developing malignant melanoma was seen (odds ratio (OR) 1.8 for almost always vs never using sunscreens)."
http://www.ncbi.nlm.nih.gov/pubmed/7734957